Hi, Susan!
You ask a very good question, and one which guests don't give much thought until they are on board. It really is important to keep your stateroom key with you at all times -- most importantly for the adults in your traveling party.
Your stateroom key has much information printed on it and carries much information "in" it, too. Disney Cruise Line keeps track of guests by checking them on and off the ship at each port of call using each stateroom key -- including the smallest cruisers.
Of course, you know your card gives you access to your stateroom, but it's also your way of keeping track of photos taken by the many photographers on the ship as well as at Castaway Cay. Plus, if you find yourself in need of some supplies -- such as sunscreen, a hat, sunglasses or a cocktail -- you're ready. We also didn't realize until we were on the island that the various members of our group were going to want to do such a variety of activities. My daughter and husband ended up renting bikes and going on a bike ride while my teen son rented a kayak. My little guys and I were content renting a few rafts and inner tubes and chilling at the beach. All those rentals can be charged to your stateroom using your card. (And there's Castaway Cay exclusive merchandise that will be really hard to resist, too!)
In addition, there's information on your card which is very helpful including your dinner time and table number, your reservation number and your assembly identification for the mandatory lifeboat drill. It's just a good idea to keep it with you always. I hope this helps you plan!
Have a wonderful day and a magical vacation. Bon voyage!
Andi