When I pull names off the database for mailing, it's very easy to include colleagues at my work address. Sometimes I forget to weed them out of the results. And so, our latest mailing included one item sent to a colleague in the same building as me. Except that he no longer works here.
And so the envelope disappeared into the mailstrom that is the Royal Mail ... round and round it went ... until finally it bobbed up back here. The receptionist saw that it was addressed to someone who no longer works here, failed to spot the strangely reminiscent sender's address, slapped a "gone away - return to sender" sticker on it and put it back into the post.
Round and round in the Royal Mail it went ...
Until it got back here. Again. This time the receptionist saw that it had a "gone away" sticker on it so gave it to me, in charge of the database, to deal with.
This is why we need the post office.