As others have said, it's a good safety net against unpredictable travel delays if you can fly in the night before. But there is another reason as well -- enjoying your first night on board. Which is especially important if you are on a 3 night cruise.
You don't say where you are flying in from but I am assuming with the time needed to check into flights and get through airport security that you will have an early morning if you are flying in the day of your cruise and arriving at 11 am. (I hope that's the first flight out from your home airport.) Factor in getting off the plane and getting down to Disney Transport check-in. If all goes well, you may be on a bus and moving at noon. With a one hour trip, you'll be at the port at 1-ish. Having to go through security and check in (if you do everything online in advance, that speeds it up). You'll be on board at the earliest 1:15. You have a few hours for lunch, touring the ship and settling in your stateroom before muster drill at 4 and sailaway shortly thereafter. If you have been up since early morning you may need to factor in a nap or miss part of your first night on board.
We go the night before to keep the stress of traveling to a minimum. If we were going on a 3 night cruise, I would want to make sure I'm well rested to maximize my time on board too.