Sea Pass cards for those under the legal age are punched so the bar staff immediately knows whether you are under 21.
I've also seen them ask for a photo ID when the Sea Pass that was being used wasn't punched and the holder didn't look old enough - as in someone "borrowing" an older pals card so they could buy drinks. On the Liberty Of The Seas in June, I saw a guy get busted by the bartender for using another person's card. The bartender held the card until the actual owner came and got it. He also got an earful from the bartender.