When big bands began featuring soloists, their spots were short. You would get eight bars if you were lucky; that was your chance to sell or go cold, and it was the only way to get your name on the marquee. Because the stakes were high, solos were assigned in advance and the players wrote out the lines they would play. Improvisation was too risky.
But that's its allure. Few art forms allow the audience to witness creation. Writers need courage to publish works even after months of revision; for a writer, the idea of someone watching over your shoulder while you write is absolutely petrifying. Jazz musicians stand onstage and bare themselves to the audience -- not just their work, but the actual process of creating it.
I've seen all kinds of bands play live -- road bands, big bands, students, and legendary players meeting for the first time. Some were consummately professional, and some weren't worth the price of admission. But I've found that the memorable concerts always left me feeling like I had been sitting in on a rehearsal.