Learning to make/compose music.
I's very rewarding to listen to your own music, mostly because you make the stuff YOU like.
And being able to play whats in your head on an instrument makes playing allot more fun.
And there are a some extremly valuable lessons to be learned from spending hours a day practicing.
1) I learned how to practice efficiently (which made learning porgraming languages and university for me allot easier)
2) I learned that I can do everything and it's just a matter of how much time I'm willing to invest.
3) I spend allot of time on my creative process which also helps allot with problem solving in daily life (and programming)