Blitz is just another format of playing chess, like swimming or running different distances. It is not supposed to improve your classical / general play skills, but it may improve your blitz skills.
Grandmasters and Blitz
When you play blitz, you're likely to make mistakes and blunders. Why do grandmasters play blitz when it doesn't help them improve? If it does, please give me a detailed explanation.
The fact remains that Blitz and Bullet chess does not improve your overall chess skills. Sure, Blitz and Bullet are a great way to have some fun, and yes it is still chess, but does not help your chess skills very much at all

Improving your blitz skills may be fine, but not improving as a chess player is not good. Wouldn't it make sense to play rapid to improve?
Yes but gms aren't really looking to improve by playing blitz. It's mainly for fun. Also there are a lot of cheaters the longer the timecontrol gets.

When you play blitz, you're likely to make mistakes and blunders. Why do grandmasters play blitz when it doesn't help them improve? If it does, please give me a detailed explanation.
Who says GM makes a lot of mistakes in blitz/bullet?
Mistakes depends on positions, if positions are not highly tactical, even 2000+ players dont mistake. in some games.
In this game, it is 1 min bullet that I played yesterday.
Time is important to avoid blunders in complicated and unfamiliar positions but in simple and familiar positions, we wont blunder in blitz/bullets.
https://lichess.org/Jl5T8xtj/black
And also if the game is too much tactical, people cant find accurate move even in 6 hours. (OTB TC)
Blitz also has its benefits, you can play x10 games than longer games and get more experience.
And in chess, not every single move is a turning point , good players know which move is turning point and invest most the time in such move. So 3 min blitz(6 min total) is a good time to calculate a lot of important moves in chess.
Mixed of Blitz and LTC is necessary for tournament play though.

There are several GMs who recommend playing blitz as part of learning new openings. Even at short time controls, you do get a feeling for the resultant positions, and it helps you learn whatever you've read in a book. Just playing over opening lines from a book is not even close to playing it in a competitive game. Although not quite the same thing, blitz does help in this regard; even if you're just playing offhand games it becomes competitive.
When you play blitz, you're likely to make mistakes and blunders. Why do grandmasters play blitz when it doesn't help them improve? If it does, please give me a detailed explanation.