Our family has historically gone during the slower time of year, normally in September, and we always save hundreds (or more) on each trip.
One reason they are so attractive is that many off-site hotels have nicer amenities, convenient access to the Disney Parks, and larger room options for larger families.
These rooms will offer the ability to cook some food in your own room as opposed to being forced to buy food out on every occasion.
Getting groceries for your hotel room is a great way to supplement your trip and minimize the expense of getting food in Disney for every meal.