I think the percentage give you a sense of familiarity. You're used to be graded 0-100 in exams and such. However, that grade is actually not that useful or intuitive as you may think, since you don't know how your game is scored. You don't understand the metric. For instance, it could mean the number of moves you got perfectly divided by the total number of moves in the game. It could be the number of moves you lost at most 30 centipawns by the number of moves. It can mean anything, the point is just that it's a black box and no one truly understands that metric outside of chess.com. Whereas average centipawn loss is a tried metric which everyone uses, except for chess.com.
