It's only needed back home When you see a Photopass photographer you just approach them and ask for a picture done, they'll then scan a card for you and away you go, use that card each time you get photo's done!
Don't worry if you leave it behind in your room one day either, just ask for another when having pictures done and they'll pass one to you
And write your initial and surname on the back of your card (or even your screen name from the DIS) - anything that may help you trace it if you lose it. Also, either take a photo of the back of the card with your camera or note the number down. The photos are not on the card, but on the system. As long as they can trace your pics with your card number, if you lose the card your pics can be placed on a new one.