Can an experienced 3 check player analyze my game?

I just played a 3 check game. I was playing as black. I made a lot of blunders, but I didn't understand the computer's recommendation. The analysis says I should have played 18. Rc7. Doesn't that give away a check with no compensation? Also, why was 21. Bxg2+ a blunder if I got a free check? Of course, I know that stockfish is right, but I don't understand why. I would really appreciate it if someone could shed some light on these recommendations.

Here is the game:

Well the game seems to be perfect

7 Ba6 accomplishes nothing productive
I prefer 13 Be7 to b4, as the bishop is a nice defensive piece, and it prepare things like f5. It also gets the bishop away from Rc1 things.
15 h5 is bad... it does absolutely nothing other than give away an hpawn. Rad8 is better, gaining control of the d-file and attacking d3.
17 Bb7 is bad as it allows white to take the 7th rank with tempo.
19 Rc8!! is a smart move as Rxb7 is met with Rc1. e5 is not good as it is met with Bxh5 (although that didn't happen in the game).

@TCF_Namelecc

Thanks for the response. That made things much more clear.

Also, I had another question. When are you supposed to sacrifice pieces for checks?