Steam Trading Cards

They give Valve more money*

But I mean, even if you work and have hobbies and friends sometimes you play games too, or you wouldn't be a member of a gaming forum. It gives both valve, you, & (sometimes) the developers free money, and entertains those with mild OCD or collection/imaginary status symbol fetishes. It's not really that bad to have the option, or ability for the devs to include them. No one demands cards from games, same as achievements.

They're given when you're playing the games you own, if you think playing games is a waste of time then why buy them ?

I'm not knocking their intention, simply the premise that there is viable "profit" to be had post-beta. I should have illustrated, my bad

The first link in Google answers all those questions

Well, you don't need to buy them per-say. The idea of Trading Cards if to trade the ones you don't need for the ones you want. It's more or less a community game. You don't need to do it. Apart from the stuff mentioned in it's description on Steam or... y'know, the fact that you can just sell all your cards, there is no other benefit. :)

Too many questions can you get killed O_O (they're watching us)

They're inflating the value of the cards (and thereby the games featuring cards), not the currency itself.