Why would a U.S. Passport be useful? Why might I need one?
If you have a U.S. passport, you can lawfully travel outside the United States and return home by air, sea, or land. A passport can be useful if you need to visit family abroad, especially in case of an emergency like illness or death. A passport also counts as a government-issued photo ID for all purposes. For these reasons, if you are eligible OR when you become eligible for a U.S. passport, it is a good idea to get one!
U.S. Passports & Int’l Travel, U.S. Dep’t of State, http://travel.state.gov/content/passports/english/passports/FAQs.html. ↑