This link makes it look like it's some event-specific website. It's not. It's an easy way to navigate the confusing mail routing system to military service members, especially those in bootcamp where they can't have phones. But you want to send them a letter which they CAN get, but you don't know their address or you don't want to actually buy stamps and go to the post office and whatever else. It's a convenience service.

Not sure why other commenter suspects people will go apeshit over this, it's just a good business idea?