Certain restaurants require payment upon booking, like Cinderella's Table, Hoop de Doo, or events like fireworks parties. If you have to cancel you will be refunded.
Not all restaurants require payment up front, but they do require a card on file - because if you are a no show you are charged $10 per person (only if no one shows up - as long as at least one person shows up, you won't be charged the no show fee).
Editing to add: When you say, "what if we have to cancel for some reason?", if you meant cancel the trip not just cancel the dining reservation, don't forget that you have to individually cancel each dining reservation if you have to cancel your trip. Wanted to cover all bases b/c I wasn't sure if you meant cancel the reservation or cancel the trip.