Since the beginning of the Post Office and the Postal Act of 1792, certain types of mail have qualified for lower postage through preferred rates. It was assumed that these types of mailings yield social benefits for senders, recipients, and more importantly, a large nation. Preferred rates’ roots trace to the first federal postal policy, which recognized that disseminating newspapers at below-cost postage would advance the important social goal of educating the electorate. Soon after, magazines received special rates. For its first 50 years, the Post Office was predominantly a newspaper circulation service, because of the high cost of sending letter mail. (Sending a one-sheet letter 500 miles cost 25 cents, while sending a newspaper that distance cost only 1½ cents.)

via A Brief History of Preferred Postage Rates | USPS OIG.