Another good reason for reviews is it lets you calm back down. Some games can alter your mindset in the next game if you just gg, and hit automatch immediatly.
For example, if you played someone who wouldn't protect their groups and the board collapsed into fighting everywhere, and you lost. You can feel cheated, he should have died.
Then if the next opponent is playing very solidly, you might still be in a bloodthirsty mood(especially if your on tilt) and make overplays that get your groups killed.
Taking the time out to review can return you to a balanced state for your next game. (Maybe)

You should play time controls that leave you enough time that you are able to actually put thought into your moves, otherwise you are just playing by instinct and it is hard to learn if you are not thinking about your moves.

I would recommend that if you really want to learn, you should review your games and try to understand what was good and what was bad and why you won or lost. It is extremely good if you can get a stronger player to review the game with you and offer comments.