Many people don't want to spend the money to rent a car especially when they are not leaving Disney Property, and Disney does have some of the best transportation you will ever see, but I have found it much easier and a big time saver to have a rental car for some destinations. If you have ever made reservations and you were running late the car will save you. Also when visiting parks and don't want to pack everything you think you may need on your back, you can leave it in the car. The last reason is that to go from a park to some other destination otherthan back to a resort you must go to the closest resort and wait for the bus to your destination (example MGM to Down Town Disney). You can survive without a car but it makes life easier and your time from one place to another faster. Remember as a Disney Resort guest you have free parking at all Disney locations.
Aloha
Steve