The unpopular old stuff will still be seeded when you want it a couple of years later, since (almost) everyone that has downloaded it and hasn't thrown it away

The unpopular old stuff will still be seeded when you want it a couple of years later, since (almost) everyone that has downloaded it and hasn't thrown it away want's their ratio's up. They won't stop seeding it until they remove the files. So the main reason to do it is high download speeds on old unpopular torrents.