Well, of course all purchases aboard the ship are made with your cruise card. It is just how that cruise card is "secured" - either by giving them a credit card number or by leaving a cash deposit with them when you board the ship.
The cash option can be a problem, because you have a definite spending limit. When the deposit runs out your card will stop working and you'll have to go to the guest relations desk and fork over some more cash. And you also have to deal with a line on debarkation day to settle up your account.
On the other hand, a credit card is very painless. Provided you have enough of a credit limit cushion, you don't have to worry about your cruise card turning into a pumpkin. And if all the charges appear to be in order (they put a final statement under your door on debarkation morning) then you need do nothing more. They will charge your credit card for the total you ran up and you avoid going to the desk on debarkation morning.