My mailbox is in a block of other mailboxes that is fairly far from my home (say, 500m) and out of wifi range without an especially large antenna. I was thinking of making a small battery-powered widget to tell me when the mail arrived and adding it to my Home Assistant (https://www.home-assistant.io/) installation.

Another use may be something like a device that warns if a locked, remote, storage area is accessed. In fact, almost any alarm system that lacks a cellular backup could add one pretty cheaply with this.