Why do most non-US Hotels ask you to post your key?

This one has baffled me for years: Why do European and Asian hotels typically ask you to post your room key(i.e., temporarily turn it in to the front desk) when you leave the premises?

I thought maybe it had something to do with preventing loss of the key, or maybe letting the maids know when the room was available for cleaning, but I finally asked the question at Reception, and there is an even more important reason.

Think about it, I'll post the answer in a blog or two.