Even if you're improvising, the fact that beforehand you know certain things will work helps you make those improvisations successful. It really helps to have a certain amount of knowledge about musical structure.
“Every time I listen back to solos of mine I'll hear something I like and then another phrase that I can't stand. You have to live with what you play. And the recording medium puts that on us. When I play live gigs I don't think so much like that.”