Eh if you collect a set you can craft a game specific badge, which levels up your profile, and get profile backgrounds, forum emoticons and upgrades to your account, stuff like extra friends list, etc.
OR you can sell them on the market place, normal cards usually go for about 10 to 20 cents, foil ones are usually a dollar or so.
It's not a lot but you can make enough to pay a portion of a game's price or even get a free one if somethings on sale and you've sold a bunch of cards or got lucky with a few foils.