Reasoning: It's a great way of re-selling an old artist 'back from the wilderness' but with nothing new on offer.
Who first: I bet that's going to be harder to nail down - there's been a huge resurgence in old acts coming back to do their old stuff, probably driven by the success of a million tribute bands around the world.
For the past 15 years or more, it's been possible to see [not]ABBA Live or [not] the Beatles, [not] Freddie Mercury, etc etc on stage in a thousand clubs round the world, day in, day out.
Some of them even come with [vaguely] amusing names, Blowasis, Bjørn Again, The Bootleg Beatles, etc
My guess is some of the original acts thought they could cash in on that resurgence. Why pay to see Loo Read when you can go see Lou Reed... why see Bjørn Again, if Abba were actually on tour, Pearls Jammin' if there's Pearl Jam. [Freddie gets a bye on this round, not being with us any more, but even Queen + Guest Vocalist have been doing the same thing.] Pink Floyd actually made up with Roger Waters to play at Live8.
Touring used to be just a way of advertising the latest album; no-one ever made money on a tour, it was a business expense.
Since the Stones first did it in the 90's & made a killing, it's now a way of making money. No more the 'tenner a ticket' in hopes of selling a few more copies of the new album, now you can be paying 100 - 150 for the privilege of taking your binoculars to try see who's way out there on the stage in the distance.
Big gigs are big money these days.