You can sell cards you get from playing games that you recently purchased (up to 3 or depending on the game), or for free-to-play games like Dota , you get cards by buying items or any in-game purchase. You can create badges with them if you collect all cards in the game, but that's gonna be complicated since you only get half the cards; meaning you have to either trade for the others or buy them at the Steam Market. You also don't get a guarantee that the cards you WILL get in the games would be all different. You can also sell the cards on the Market yourself, since there are others trying to complete their set. It's just a few cents each but I guess if you do it enough it will probably be enough for a decent game on sale.I didn't even know that was a thing you can do. I've only had Steam for a few years and I hardly use it. I have games that I mean to play, but gaming on a PC feels weird.
Have a look at Steam Community :: Steam Trading Cards if you want to know more about Steam trading cards and what you can do with then.I've got a whole bunch of cards that I've collected over the years, but I've never quite got the purpose of them. Every so often I get the green notification in the top right saying I have new things in my inventory, click it to stop it being highlighted then head back to my library.
Have a look at Steam Community :: Steam Trading Cards if you want to know more about Steam trading cards and what you can do with then.