How do you control your opponent when using a specific opening

Nevertheless, most people don't play a5. Most of the replies will be e5 c5 or Nc3.
However, if you played d4 as the first move, there are not too many reply options the opponent will consider. In most cases, they will just respond with d5. This kind of controls your opponent, but it will not guarantee that they will respond this way. Again, they might respond a5 just to lead you into dark lines that you will not be familiar with.

It's like that episode of the Simpsons. Whenever they play a move you don't want, they get an electric shock.

Excellent question.
Different openings have different typical plans. In the Spanish, both sides focus on the centre above all else, often with lots of slow maneuvering, and the game is very strategic. In the King's Gambit, the focus is more on the Kingside and things happen much faster. In the King's Indian, White tends to play on one side of the board and Black plays on the other; whoever is faster emerges victorious.
People play various openings because they generally enjoy the associated plan. Most people who play the King's Gambit do so because it gives them positions they like. As amateurs we mostly play to have fun, and if we can get positions we like then we will have more fun, right?
Also, different openings can keep chess fresh. Rather than always playing the same opening moves every time, we can mix things up. Within each opening they are different variations, and these variations have sub-variations. Each one subtetly changes the nature of the position, and if you understand this you can tailor openings to fit your preferred style or mood, as the case may be.
At the professional level, it is necessary to know these ideas in very minute detail, hence the 10 pages of analysis you've mentioned. For non-professionals, this is unnecessary. It can be fun, though, to play the same positions as Kasparov or Fischer or any other chess hero, to follow in their footsteps.
"... there will come a time, whether on move two or move twenty, when your knowledge of theory runs out and you have to decide what to do on your own. ... sometimes you will leave theory first, sometimes your opponent. ... It happens in every well-contested GM game at some point, usually a very significant point. ..." - IM John Cox (2006)
"... For beginning players, [Discovering Chess Openings by GM John Emms] will offer an opportunity to start out on the right foot and really get a feel for what is happening on the board. ..." - FM Carsten Hansen (2006)
https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf
"... Overall, I would advise most players to stick to a fairly limited range of openings, and not to worry about learning too much by heart. ..." - FM Steve Giddins (2008)
"... I feel that the main reasons to buy an opening book are to give a good overview of the opening, and to explain general plans and ideas. ..." - GM John Nunn (2006)
"... If the book contains illustrative games, it is worth playing these over first ..." - GM John Nunn (2006)
"... the average player only needs to know a limited amount about the openings he plays. Providing he understands the main aims of the opening, a few typical plans and a handful of basic variations, that is enough. ..." - FM Steve Giddins (2008)
"The way I suggest you study this book is to play through the main games once, relatively quickly, and then start playing the variation in actual games. Playing an opening in real games is of vital importance - without this kind of live practice it is impossible to get a 'feel' for the kind of game it leads to. There is time enough later for involvement with the details, after playing your games it is good to look up the line." - GM Nigel Davies (2005)

Stay active. Make sure your moves are purposeful. Study variations, recognize patterns. For me it is all about knowing the resulting opening variations better than your opponent. How can you pull your opponent into a line that you know better than him? If you've played out a sideline dozens or hundreds of times and your opponent is playing the position for the first time--or whatever--you have a major advantage.