USPS

[crashdown]

Because they do things like pre-sort their mail etc, saving the post office from having to do it. The original concept came from "You're sending carriers on those routes anyways, if you give us a preferential rate we'll give you more mail, have it presorted and ready for the carriers, etc."

This and many other factors.
If it wasn't for "junk mail" I would bet stamps would be 20-30 bucks each and packages would cost more in postage than the contents of the box 99% of the time.
"junk mail" keeps the USPS alive, and it hardly feels like a break when you see 20-30K going to the post office every week.... oh, and "junk mail" just got hit with a 7% postage increase this month.
"junk mail" is also delivered to the local post office, not picked up at the printing facility.