Because they're rare, and they grant you powers beyond human comprehension.

I can give you one if you want

I think they're one of the prizes if you collect all the cards for a game, but only if you're steam level 10.

Because if you craft a set of foil cards you will get a nice foil badge featured on your profile, and you can finally be the big bragging friend that everyone wants to have on Steam.

it is useless stuff that everyone thinks they need.

ALIENS

Foil Cards = Foil Badges

Because Gabe wants your money. Obviously.

It's absolutely useless but expensive.