A key, when reduced to its simplistic definition, is used to unlock various things. Most keys are used on doors. Allowing access to new places and disallowing access to those not permitted.
A cog is not as imperative as a key. It does not have the power of opening said door but rather helping it swing open. Much like the ball hinges that allow that motion to occur after the key is used to actually open, its job is important but not imperative.
After unlocking the door you could simply push the door down without the hinges but it is much more helpful and graceful when the hinges actually exist.