The permanent change of address triggers advertising that targets new residents. Restaurants want to try to snag your regular business before you find other options on your own. Hardware stores expect that you might be doing some post-move remodeling. Insurers might try to sell you on a new homeowner or renter policy. Cable and phone companies offer you limited-time deals for new customers.
It's a one-time deluge that tails off pretty quickly. The USPS makes a lot of their revenue from periodicals (2nd class) and marketing (3rd class) mail.