I got one because my passport was getting beat up from being in my pocket on flights all of the time. If I am on a plane, my regular wallet, cell phone and passport (if international) stay on my person at all times. It is also a handy place to keep leftover foreign currency.

I figure if we have to ditch in a river somewhere and my carryon gets left behind, I still have everything I need to survive (cash, credit cards, phone numbers) and travel (passport, drivers license). I keep a backup credit card in the passport wallet in case I lose my regular wallet.