To send and receive USPS mail, you need... a mailbox. To send and receive email you need an internet subscription and a computer. Internet access can be impossible to find, or very expensive, in many areas that are serviced by USPS. Saying "It doesn't cost anything to email someone... who is across the globe" is only true if you both have the infrastructure, equipment, and service necessary.
Well, to receive USPS mail you need an address. Permanent travelers, the homeless, people in the process of moving, moving- these people are difficult or impossible to reach by mail, but any time they pop into a library or internet cafe, they have an email address. I would say that /that/ is the great equalizer, more than remote people with mailboxes in this day and age.
So just by writing GENERAL DELIVERY instead of a street address, it gets held? I suppose the kick here is that the person needs to know they're getting mail. Do they need an ID to pick it up?
It's rapidly becoming that you only need a cell phone, which would potentially make the internet much more feasible than snail-mail; owning a cell phone is far easier than owning a house and staying in a single location.
I'll grant that it's not there yet - plenty of places aren't covered by cell towers. But they're diminishing, and plenty of places aren't on postal routes either. If anything, cell towers will soon cover more area, as I can get reception miles from any road, but USPS won't deliver to where I'm standing.
