Or maybe you're forgetting that the postal service delivers more than just spam and circulars? (Also, you're picking local grocery circulars as your example of spam? I would wager that grocery circulars are among the few pieces of unsolicited bulk mail that most people don't consider unwanted.)

The postal service also delivers tax documents, ballots, bills, and (actual and important, not the junk made to look as such) notifications from utilities. It's disingenuous to argue that the postal service doesn't do anything important.