Most cases of people paying to see human performers are of performing already-written music though, so I doubt think that's the drawcard.
Actually I don't think anyone truly knows why we obviously gain something from watching live performers that can't be obtained from watching them even on the best quality home entertainment system imaginable (*), but at least part of it is feeling like you're part of the performance, knowing that the performers feed off the reactions of the audience etc.
(*) And why has there never been a market for broadcasting music performances in cinemas?
