USPS Box For Rent

On a street corner in Chicago I saw this USPS letter box labeled “For Rent”.

The sign said nothing but, “For Rent” and it provided tear-off strips of paper with a phone number.  No more detail provided.

I admit to being curious. What is for rent? The mailbox? What for? Can’t everyone use it already? Would you get exclusive rights to using the mailbox for your post? Perhaps it would be a good “dead letter” box if you happen to be a spy?

Why did it take two sheets of paper to make this sign?

If this sign listed a website instead of a phone number I would definitely check it out.