Why does everyone want tf2 keys and Paypal?

TF2 keys and paypal act as currency and make it easier to find a match. I will happily trade a game I have for an equally valuable game I want, but it can be damn near impossible to find someone who has what I want, wants what I have, and agrees with me on the value of the games. Far, far easier to trade my game for a generic form of currency (like keys), and then trade those keys for a game I want.

To me it's just stupid, I don't use keys, it may make things easier, but don't you have to pay to be able to trade them?