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Because that's the cheapest way to obtain new Hats.

Now that i think about it, may be the stable price, since it's being constantly sold.

Trading. It's the most commonly used trading currency.

It's something that does not change much in value, that has a constant supply, and that can be traded. Keys work like money does in the "real" world (so you can assign a "key" value to games).

keys open boxes, boxes contains stoof ,people wants stoooof ,people want keys.

surprised me at the beginning of my steam entry as well ... (that the price doesn't fluctuate wildly)
whats already been mentioned + the safest way trade with steams trading window without trusting
the other party and to cash out safely very expensive/rare csgo-skins (keys <> skins -> keys <> money)

Think of it as a currency. 1 Euro= $1.1175. 1 key= $2.70

Because
FABULOUS
FANCY
(ﾉ◕ヮ◕)ﾉ*:･ﾟ✧

moneys

It's a trade Item that's almost untracable, and since you can't cut them in half, they're perfect for greedy D-bags juicing more out of a trade.

both games all about hats , skins and keys

The same reason people buy Apple products or any overpriced item: because marketing told them that it is good. And for some it is just a simple collection thing. Some people on Steam collect games they never want to play, some collect cards for levels that are practically worthless (unless you are a big-shot streamer or similar online gaming personality, I highly doubt you need 500 friend slots, especially Steam groups let you have as many invitable buddies as you want), some collect pointless 300-dollar CS:GO skins or pointless 4000-dollar TF2 hats.