[Symbiote]

At some point in the past, the balance of intracity vs intercity mail would have been different.

Now, utility bills come from some distant office, as do letters from banks, and we don't send invitations very often.

X years ago, bills were sent by closer companies, the bank was local and it was polite to invite someone to dinner by letter.

With that, it would have made sense to separate local and other mail as soon at the postman brought it to the post office.