The United States is a vast country, with great diversity in environment of living from high density urban areas to remote villages only accessible by plane or boat.
Many companies refuse to ship to P.O. Boxes, and in some cases even to 17.5% of the country (Alaska). This is a practice that should be forbidden.
Some people live in places that do not have registered street names or addresses, and thus have to rent a P.O. Box in order to receive mail and packages. Many have no access to a lot of products and services other than via mail/parcel delivery. Then there are of course people who rent a P.O. Box for privacy and security reasons.
Companies should be legally obligated to ship to P.O. Boxes, and companies should not be charged extra for doing so by USPS.