the middle game of a chess game is a lot like the middle of an oreo cookie. you wanna open it up and lick inside
What is the point of a middle game?

In the opening you bring your pieces off the back rank. In the endgame it's about technical execution. The middlegame is about what happens between bringing pieces are off the back rank and the technical phase... so lets focus on their evolution. Their 4th stage is what will answer your question.
The bird's eye view of a piece's life has 4 stages:
1) Basic development
This should be self explanatory at your level. Bring pieces off the back rank.
2) Mobility.
Pieces that influence more squares tend to be better. This means the center is important, but also be aware of friendly pieces blocking each other. A good example is a knight on f3 blocking a bishop on g2. Often an early middlegame maneuver involves re positioning the knight. A well known example is the idea of a "good" vs "bad" bishop.
3) Infiltration
Mobility is usually not enough. You often need to cross the mid line and post pieces in the enemy's territory.
4) Where your pieces come into contact...
with weak enemy pawns or the enemy king. Pawns and the king are the slowest, and therefore the biggest strategic targets. This is the whole point of the previous 3 steps... you can ignore one of them, or all of them, so long as you satisfy #4 which is pressuring weak pawns or the enemy king (skipping some or all of the previous steps sometimes happens in very dynamic openings).
And from a bird's eye view, that's what the middlegame is all about: piece activity... the ultimate goal of which is usually coming into contact with weak pawns (a more strategic game) or the enemy king (a more dynamic game).

I guess that only answers what and not how... and answering "how" is pretty important.
In general after the opening you will select one of 3 areas to seek play: kingside, center, or queenside.
To facilitate this often involves a pawn break. If you're not familiar with the term "pawn break" then google it. In short it means opening lines (files, ranks, diagonals) for your pieces.
You should pick the area where you have better development or more space (space in this case meaning your furthest advanced pawns are further than theirs).
In both cases (development and space) you're choosing a side where you have more mobility... which means better chances of infiltration and ultimately satisfying #4.
Other times games are very equal and you spend a lot of time in #2, and you can't really choose a side yet... but being aware that you'll eventually need to focus on an area may help you as you continue to gain more experience and skill.

Thank so much for the useful comment, I'll definitely think of middle games differently now @ llamonade2

I'm glad to help
It took me over 10 years of playing to arrive at some of these conclusions...
And books written by GMs, some of which define coordinating your pieces as "coordination is when the pieces work together" are staggeringly superficial... so I'm really happy to offer an real explanation.
In their defense they're native speakers while we're trying to learn a 2nd language. If you ask a native English speaker what a conjunction or homophone is, they'll probably have no idea. They do it intuitively. GMs develop plans so intuitively they're (sometimes) incredibly terrible at trying to explain what's really going on to the rest of us.
Here are some reading possibilities that I often mention:
https://www.chess.com/article/view/how-to-play-positional-chess
https://www.chess.com/article/view/test-your-positional-chess
https://www.chess.com/article/view/do-you-really-understand-positional-chess
https://www.chess.com/article/view/get-ready-to-test-your-positional-chess-again
https://www.chess.com/article/view/can-you-pass-this-positional-chess-test
https://www.chess.com/article/view/how-to-calculate-long-term-advantages-in-chess
https://www.chess.com/article/view/learning-basic-pawn-structures
https://www.chess.com/article/view/learning-pawn-structure-for-chess-players-under-2000
https://www.chess.com/article/view/your-pawn-structure-is-your-friend
https://www.chess.com/article/view/more-pawn-structures
Simple Attacking Plans by Fred Wilson (2012)
https://web.archive.org/web/20140708090402/http://www.chesscafe.com/text/review874.pdf
http://dev.jeremysilman.com/shop/pc/Simple-Attacking-Plans-77p3731.htm
Logical Chess: Move by Move by Irving Chernev (1957)
https://web.archive.org/web/20140708104437/http://www.chesscafe.com/text/logichess.pdf
The Most Instructive Games of Chess Ever Played by Irving Chernev (1965)
https://chessbookreviews.wordpress.com/tag/most-instructive-games-of-chess-ever-played/
Winning Chess by Irving Chernev and Fred Reinfeld (1948)
https://web.archive.org/web/20140708093415/http://www.chesscafe.com/text/review919.pdf
Back to Basics: Tactics by Dan Heisman (2007)
https://web.archive.org/web/20140708233537/http://www.chesscafe.com/text/review585.pdf
https://www.chess.com/article/view/book-review-back-to-basics-tactics
https://static1.squarespace.com/static/5856bd64ff7c50433c3803db/t/5895fc0ca5790af7895297e4/1486224396755/btbtactics2excerpt.pdf
Seirawan stuff:
http://seagaard.dk/review/eng/bo_beginner/ev_winning_chess.asp?KATID=BO&ID=BO-Beginner
https://web.archive.org/web/20140708090229/http://www.chesscafe.com/text/review492.pdf
http://www.nystar.com/tamarkin/review1.htm
https://web.archive.org/web/20140708092617/http://www.chesscafe.com/text/review560.pdf
50 Essential Chess Lessons by Steve Giddins
https://web.archive.org/web/20140708100833/http://www.chesscafe.com/text/review534.pdf
http://www.gambitbooks.com/pdfs/50_Essential_Chess_Lessons.pdf
Amateur's Mind by Jeremy Silman
https://web.archive.org/web/20140708094419/http://www.chesscafe.com/text/ammind.pdf
https://www.silmanjamespress.com/shop/chess/amateurs-mind-the-2nd-edition/
Simple Chess by Stean
https://web.archive.org/web/20140708104258/http://www.chesscafe.com/text/review400.pdf
http://store.doverpublications.com/0486424200.html

I guess that only answers what and not how... and answering "how" is pretty important.
In general after the opening you will select one of 3 areas to seek play: kingside, center, or queenside.
To facilitate this often involves a pawn break. If you're not familiar with the term "pawn break" then google it. In short it means opening lines (files, ranks, diagonals) for your pieces.
You should pick the area where you have better development or more space (space in this case meaning your furthest advanced pawns are further than theirs).
In both cases (development and space) you're choosing a side where you have more mobility... which means better chances of infiltration and ultimately satisfying #4.
Other times games are very equal and you spend a lot of time in #2, and you can't really choose a side yet... but being aware that you'll eventually need to focus on an area may help you as you continue to gain more experience and skill.
weak squares, too...

Ignore these guys massive messages.
Middle game is about optimising all ur minor and heavy pieces. In short this means u want to think about what ur plan is from the middle game till end, and put ur pieces in squares that give them access to execute that plan.
That's literally it. besides the obvious of protecting ur pieces, no blunders etc. Ur an amateur as u said, no need to complicate it, just keep that idea in ur head.
Well then, what is a plan? I think these guys got it perfectly. The strategies they gave were examples of plans.

Ignore these guys massive messages.
Middle game is about optimising all ur minor and heavy pieces. In short this means u want to think about what ur plan is from the middle game till end, and put ur pieces in squares that give them access to execute that plan.
That's literally it. besides the obvious of protecting ur pieces, no blunders etc. Ur an amateur as u said, no need to complicate it, just keep that idea in ur head.
Yeah, but what you wrote doesn't say anything. It's just truisms. Someone who's never played chess could write what you just wrote.
For example I've never painted a picture in my life, but I could write some blah blah blah about "painting is about putting the paint on the canvas in the optimal way. It's about brushstrokes that define shapes with color and form. You should think about what you want your picture to look like, then start at the beginning and go step by step until you're at the end"
Totally vacuous statements...
lol

Ignore these guys massive messages.
Middle game is about optimising all ur minor and heavy pieces. In short this means u want to think about what ur plan is from the middle game till end, and put ur pieces in squares that give them access to execute that plan.
That's literally it. besides the obvious of protecting ur pieces, no blunders etc. Ur an amateur as u said, no need to complicate it, just keep that idea in ur head.
Yeah, but what you wrote doesn't say anything. It's just truisms. Someone who's never played chess could write what you just wrote.
For example I've never painted a picture in my life, but I could write some blah blah blah about "painting is about putting the paint on the canvas in the optimal way. It's about brushstrokes that define shapes with color and form. You should think about what you want your picture to look like, then start at the beginning and go step by step until you're at the end"
Totally vacuous statements...
lol

Yeah, potato Jesus right? Botched restoration by amateur artist that made some small village famous IIRC.

Paintings different from chess. U can't teach someone how to effectively play the middle game. All u can do is give advice about how to enable them to do it if they develop a proper plan. I actually made a post about the middle game a couple years ago, and all the advice on there never helped me. Simple advice like optimising ur pieces for the plan u have is the only advice u can give.
I agree with your general point. They have to do the learning themselves, and reading prose is low on the list of useful things to do.
Still, I think introducing some ideas (especially emphasizing piece activity) plant useful seeds. During the process of gaining more experience they can look at their games (and other's games) through lenses like piece activity and it'll help them extract useful lessons.
... U can't teach someone how to effectively play the middle game. ...
Is effective-middle-game-play a yes-or-no thing or a matter of degree? Is there reason to rule out possible improvement with the right sort of book?
"... In many ways, [Batsford's new edition of Logical Chess: Move by Move, written in 1957 by Irving Chernev] would [be] a wonderful 'first' book (or first 'serious' book, after the ones which teach the rules and elementary mates, for example), and a nice gift for a young player just taking up chess. …" - IM John Watson (1999)
https://theweekinchess.com/john-watson-reviews/assorted-recent-books
... U can't teach someone how to effectively play the middle game. ...
Is effective-middle-game-play a yes-or-no thing or a matter of degree? Is there reason to rule out possible improvement with the right sort of book? ...
U realise just recommending books isn't gonna make someone improve right lol.
I am just trying to call attention to possible choices.
U think someone that plays casually on chess.com is gonna go read a book about chess? lol
I have no idea what an individual is going to do. I see nothing wrong with calling attention to possibilities.

The middle game is where sharp lines become downright lethal. Hand to hand combat, to the victor goes the spoils.
For more moderate lines, the middle game is where subtle moves can have lasting impacts on the game. Regardless of the opening variation, the middle game is where players have the most opportunities to find tactical gemstones just waiting to be plucked out of thin air.
I really enjoy playing chess, and I am trying to improve my game. I have done quite a bit on openings and the endgame but I've left my middle game lacking. Firstly what is the main goal of a middle game. And secondly, any advice for an amateur chess player on middle games?
Thanks.