One of the perks of being a member of Congress is access to a “free” mail system. Senators and representatives can send information to constituents about legislation, congressional votes or other government-related matters and “frank” the envelope, with the taxpayer picking up the stamp. (It’s not really free because you and I pay for it.)
The franking system isn’t for personal mail. If you’re a member of Congress and want to drop Aunt Gertrude a note to ask how her gall bladder surgery went, you must pay for the stamp yourself.