Muso commentator and critic Graham Reid at Elsewhere recently asked how come live tribute acts are ever more popular when you can get the original music so easily? Is it the thrill of the communal live experience of the "ultimate pub band"?
I think two things are going on here:
These acts present a snapshot of a band or artist in their formative years before they became mega famous, distant, and shackled to forever playing their hits.
The very act of tribute produces musical performance and audience response that is very different from the original playing.
And Graham is right - the live experience is quite special and lives on beyond artist retirement when picked up and done with energy and respect.
Sometimes the tribute is better than the original!