I can think of a few reasons one being a firm place for the chair legs, it avoids stress on the frame of the chair.
Another reason avoiding the pest on the ground, ants, snakes, scorpions, spiders, fleas, ticks and such.
Can be amusing to do a bit of...why dat? thinking trying to get into the other fella's head.
In this case I would say they might just be doing it for a sensible reason, those biting ground dwelling creature can sure be the cause of some nasty health issues. If you want to relax and not be bothered by those critters and you have a good above ground platform for a chair I say go for it and don't worry about other people wondering why you are not down on the ground.