Steam Trading Cards

You can use them to make badges and boost ur steam lvl theirs some other things u can do but im not sure on all of it

Craft them to create a badge, each badge gives you 100XP, XP grants you steam level, steam level grants you higher chances to drop booster packs.

What's AH?
You surely sold them cheap :( there are cards worth much more (even euros/dollars)
I'm sorry for you, you should have asked before :(

There is no point to them, they are collectible items. If you do not want to collect them, then you just pretty much have to keep doing what you did so far. :)

You can trade it for games too :) a lot of people trade good games for 5-10 cards each

You can sell them, you can trade them for something you want, or you can use them to level up. It's your call!

its all about steam level and other stuff. when you have a full cards set you can craft a game badge which raise your steam level by giving you a fixed amount of exp based on the badge level. crafting a badge also gives you emoticons and wallpapers related to that game for personalize your profile. you also get a coupon for a random (?) game.

everytime you level up (steam level), your friend availeable slot rises by 5 and every 10 levels you have higher chance to get a random booster pack of a game you are eligible of dropping them (for being so you have to play a game with cards untill you get the droppable ones, about half of a complete set) which contains 3 random card of that game

might have mispelled, hope its clear enough

Adding to all that has been said above, every 10th Steam level you get an extra showcase slot to further customize your profile.
Also, it's a common practice for traders to raise their Steam level, to make their accounts look more professional.

Free money

You can get background images and emoticons as well as badges when you combine cards.
They can be put on the profile to fit your taste. You can brag it to your friends and make yourself look trustworthy.
If you're neither social nor collectors nor interested in trades, cards will be nothing to do. They will be small coins in steam wallet after they're sold on the market.

Make Badges to boost steam level and get more ways of customizing your profile. The only real benefit to leveling imo is that it increases your odds of getting booster packs for games you've collected all card drops from.

Collect full sets, craft badges during summer/winter sales and sell the resulting sale cards, emoticons and backgrounds. Then buy tons of free games from the sale.

You can get full set then up your steam level.

Every 10 steam levels, you get an additional room in your profile to display achievement/games or whatever.

If you don't care about that, you can sell your cards to lower your game costs.