That plus the ability to deliver to mailboxes and P.O. boxes.
And that goes back to the original intent of creating the Post Office. Unlike the model that a private business would use, First Class mail pricing is independent of location or distance. The question is, is it in the public interest to make people living in rural areas pay the extra price that it costs to serve remote and thinly settled areas? Or is it in the public interest, and worth a public subsidy, to maintain such service to such areas so that rural dwellers have a relatively inexpensive way to send mail and small packages? You may not think so, but it seems to be quite Constitutional.
What carriers distinguish between rural and urban destinations? When sending packages Brown or (I think) Orange, UPS charges based on geographic distance (NYC to LA is more expensive than NYC to Albany) but I don't think there's any difference sending a package to Squirrel Island, Maine vs. Augusta, even though the former is only reachable via boat.
They may not distinguish other than on distance, but for the more rural areas they get the articles to their nearest depot, then mail them rather than go to those remote points...