The Postal Service, by whatever name it is called, is a part of national communication as much as airlines, radio, TV, telephone, highways, electricity and so forth. USPS is obligated to maintain and staff thousands of small, rural offices, and deliver virtually anywhere in the country. No private corporation has this obligation and none would show a profit if they had to do this. Thousands of tiny villages have post offices because they are part of the communications glue that binds people to neighbors, friends and families far away. FedEx will not stop by your home every day (except Sunday) to see if you might, perchance, have a letter to mail; or if there might be a package of medication shipped from a thousand miles away to be delivered.
USPS is in a difficult financial position, but privatizing it - as the current idiotic Postmaster General wants to do - is merely a greedy way to isolate rural Americans. Privatization places profit before national connection and value to the public, and is fundamentally counter to America's democratic values.