Pretty much for the same reason we would use a babysitter at home.
Our kids are 6, 8, and 10 now (they were 2, 4, and 6 the last time we went to WDW). There is a lot to do at WDW for adults. It was one of our favorite vacation spots long before we had kids. Don't get me wrong, there isn't really anything that they like and we don't like. But, unfortunately, the opposite is not true - there are plenty of things that we like and they find boring (most things at Epcot, for example).
Our youngest was too young for the kid clubs last time, so babysitting was our only option. This time we have a choice. Babysitting is cheaper with three kids, but I think they would love a place like Neverland, so we'll probably try that at least one night.