Isn't TF2 free?
Keys open crates which carry statistically low chances of rewarding the crate opener with unique items that are highly prized within the TF2 community and can actually net some surprising coin from people who collect digital trinkets like that.
You may have been thinking key that authorizes use of a game and this is not that kind of key.
tl;dr:
buy key, open crate, get random item