I don't believe that you really think twice about who you are, and nobody is likely to ask. If someone did, you have identification. But what if, as an American, you're in a foreign country, and you should lose your passport? That's your best means of identification, so if you don't have your passport, you will be detained until the US Consulate clears you. The delay is annoying and frustrating, and it could be costly, as Paul Chafin found out, in the story you are about to experience...