You'd be amazed. Ive used dreaming to help me play the trumpet(Ive played for 10 years). One night I played a mad solo in my dream, where every note was perfect, and I could tell as it happened. When I woke up I was able to re-create a section of it.

This isnt the only example you can find. Ive heard of other musicians playing entire pieces and then being able to sight read it perfectly afterward(I wish I rememebr the exact place I read this). The dream world is infinite in recourses that you can use in a million ways to help you in your waking life.