During the day, I would use fill flash to chase away the harsh Florida sun shadows especially on people or if the light is behind your subject(s). Indoors (where allowed, see below) in places like ride queues, stores, resorts, restaurants, and World Showcase pavillions.
At night, to illuminate subjects in the foreground. If your camera can do slow flash or rear synch, you can get some great effects (would require a tripod though).
Of course, many shows in WDW do NOT allow flash photography for the enjoyment of your fellow guests and/or safety of the performers.