Vehicle registrations and drivers licenses require a physical address. Just having a Post Office box does not meet this requirement. Also when you travel you may want your important mail forwarded to you and the U.S. Postal Service does not forward your mail for you.
Although you can find mail forwarding services online they are a bit costly so we chose to use a local UPS Store to handle this for us. We rented a box at the UPS Store for a year to start with. We do not have to drive there to check our mail as we can call them anytime during business hours to find out if there is mail waiting to be picked up. We also paid an advanced payment for forwarding our mail should the need arise. This payment covers the cost of postage and fees involved when mail is forwarded to another location.
Using a mail box service has another advantage over the U.S.P.S. You can receive packages and mail sent through carriers other then the U.S.Mail. This means you can still shop online and have UPS, Fed-Ex and DHL deliveries as well as items through the U.S.P.S. Also items not picked up within the U.S. Postal Service time frame will not be returned to senders. Big plus, no more pure junk mail.