Two other reasons: 1. You learn what you don't know about the topic or things you assumed you understood but really don't (a comment by @Swizec identifies the

Two other reasons: 1. You learn what you don't know about the topic or things you assumed you understood but really don't (a comment by @Swizec identifies the Illusion of explanatory depth; TIL) 2. You learn what things you thought you "knew" that are either contradictory or unfounded.