Many of the restaurants, especially those in the theme parks, will simply turn you away if they're completely booked and you don't have an ADR. They won't even let you wait for a table.
You can always call that day and try to get a last-minute ADR, but you'll have a very limited choice of restaurants and dining times, and the restaurants that you can get into may not be in a convenient location. For instance, you're planning to be in the Magic Kingdom, but there's availability at Olivia's at OKW, or Spoodles at the Boardwalk.
You can always cancel an ADR. Far better to make the reservation and cancel it if your plans change, than to be wanting a table-service meal and not be able to get in anywhere.