Whilst the model used the heater (700w) during the Wash/Rinse phases to maintain the water temperature as Bob said, the full power of the heater was only use when the "Sani-Wash" was selected, so as to heat the final rinse water to 180ºF.
Therefore, you need a nice, hot water supply to ensure the Pre-Washes/Rinses heat up the tank/contents of the dishwasher to ensure washing performance is up to scratch for the Main-Wash period.