In the traditional folk music field that I play in, some of us learn "by ear" and others learn by reading. I threw out the sheet music over twenty years ago. I know some people just can't play anything if they don't have the music right in front of them. I get frustrated with some of them, as folk-type music varies a lot with individual impressions of a song and what version of a traditional song a person performs. Someone who can only read and not play by ear can't pick up on what's going on if the song varies a bit from what's written in front of them.