This style of mailbox reminds me of visiting my grandparents in rural Missouri. Their mailbox listed their name rather than their house number, since they didn't have a number. The flag indicated whether there was outgoing or incoming mail. The mailman delivered the mail from his truck, since each farm was quite a distance from the next.
Here in Downey, the mail people deliver the mail on foot, and never have serious weather (that is, snow) to deal with. But that flag still indicates, "You've got mail."