The US Postal Service provides services that let businesses or large organizations send mail to every address in a zip code or delivery route. Much of the time, this mail is unsolicited, "junk" mail.
Some households value this kind of mail; for instance, most coupon magazines are delivered in this fashion. For the households that don't value this kind of mail, it represents a huge waste of money and resources. Mail is printed, delivered, and thrown away without ever being read.
If the USPS provided a way for households to opt-out of receiving this type of mail, it would dramatically reduce waste -- both the waste of paper, and the waste of time and effort spent delivering mail that would never be read.