the cards are for badges. Im guessing a lot of feedback about getting more badges was given. so developers have to sign up create card artwork and submit it to valve. Each card gives you experience which will level your account up. Some cards go for 20-40cents depending. but cards like the summer sale cards go for 15-18 cents right now.
on a more indepth note,
the badges that can be crafted from the cards can also be leveled. so once you collect all cards in a game set you can continue leveling the badge. however after your initial card drop, which is half the size of the set. you have to start purchasing the cards on the market. after steam level 10 you have a chance to get a booster pack drop. which is 3 random cards from any game.
its kind of fun to do. but im not a huge fan of buying the cards.