Well fans get to see up close with people involved with an artist they truly love (and they are probably to some degree fans of these musicians as well) and to interact with them more than they would be able to in another type of event.
As for the musicians, I don't know if they get paid at all, but probably not. Beyond that, I guess they get to celebrate having been a part of something big and loved by so many people. Must be a nice feeling.
Look how pleased Street was to be able to play Everyday is like Sunday live, which he never had the chance to do with Morrissey.