The ground has a lot of moisture in it (except in the desert) so if it sits over the ground the air under the car will be more humid and you'll get more condensation on the bottom of your car and it will probably rust faster over several years. On houses built over a dirt crawl space you will hopefully find a vapor barrier such as a heavy plastic on the ground under the floor to keep the humidity down and hopefully prevent the floor from rotting over the years. Same concept. If you put plastic under the car, you'd still want to keep rain from collecting under it so I'd make sure it was not on a perfectly level surface and material was tight to prevent too much puddling. I've even heard of people doing this when parking a car indoors for long periods of time as well.
