Comment by retrac
Mailboxes are bidirectional. If there is outgoing mail in the mailbox your postal worker will take it to the post office. That is what the flag is for. Not sure how well this is adhered to anymore, to be fair. I could be out of date.
Not sure about OP but it's become more common in certain areas of the US to not have mailboxes in front of each house. But usually there's a shared bank of mailboxes within a mile so with the ability to drop off outgoing