This is something I don't understand either. I don't read music and play entirely by ear. I wish now that when I was a child I had gone ahead and learned how to read music, instead of faking out my piano teacher who would play a piece through for me before we worked on it - and then I would be able to pick it out without reading the notes. Eventually, as things got more complicated I couldn't do that anymore and I was found out. My parents were kind of disgusted (can't say I blame them) and pulled me out of lessons. Mostly, I would like to read music so I could figure out melodies from song books.

But I don't understand why a musician who could read music and was technically very good at the instrument, wouldn't be able to just play (especially a simple piece) without the sheet music.   I have known this to be the case, though, with people who make their living playing music in symphonies. They can play all kinds of complicated violin parts, but they couldn't play even the most simple fiddle tune without music, and look shocked at the very idea. That said, I know some other symphony musicians who transition quite easily to traditional styles and play without sheet music.