You only get the rewards based on some certain criteria, and these rewards van be anything but I don't think they are worth what you might to spend to earn them. And I don't think being a VIP increases your chance winning every game more than a non-VIP player.
Casinos giving back in return? That's a bit shady if you ask me. They're not going to do it if there's nothing in it for them. These VIP schemes are a way to draw users and to keep them coming back. Not that complicated and it's good for the business if you have loyal "VIPs" And "high ranked" Players frequent on your site.
Casino can give certain bonus or returns to the gamblers who have attain VIP status. If they give special features for VIP members, many people will try to attain the VIP status and thus they will wager more on the site. The main objective of these special status is that they keep the higher wagering players attract towards their site.