If you arrive early, you can go to your resort and check in. They will hold your luggage if you want, and give you a phone number to call if there isn't a room ready that fits your needs. Then, you can go off to a theme park, water park, or shopping until 3:00pm or so. You can get your room number then.
The advantage is that if you happen to fly into Orlando early, or want to do something the first day you are there, you don't have to stand in the check in line in the middle of your day.