As Skylar19 said, they guarentee you a table at a sit down restaurant. I believe that if you know where you want to eat, or if you want to eat at a specific time it is essential to make ADR's. Most often you are seated within 15 minutes of your ADR time, but sometimes you do end up waiting a bit. That said, theDinig Pla had made TS restaurants very popular and many are turning away "walk ups" or they are waiting hours to be seated.
Check out the menus on allearsnet.com. They will give you an idea of the menus as well as the cost of food at Disney.
Have Fun Planning!