Hi Tom,
They are usually selectable between hot and cold fill. If you fill with hot and cold, you can choose to use the heater to boost the temperature, either to 40 or 60degC.
You need to remember that up until the 60's, you had to be reasonably wealthy to have a washing machine and an internal laundry here, generally people had a copper (Wash Boiler) that heated the water via Electricity, Gas, Kerosene, Wood Chips etc, located on the back verandah or in a detached building. This meant that when the washing machine became popular you had to be able to compensate for laundries that didn't necessarily have a hot water supply.
A lot of houses also had gravity feed hot water tanks, rather than mains pressure units (Think Open tank in the roof, a Ballcock to control water level and a submerged element in the bottom), the low pressure these operate at, means they're not always suitable for an automatic washer.
In relation to heating, you start the machine filling, add the detergent during filling to dissolve and then add the clothes. The machine then sits there silent for 60-90 minutes whilst it heats the 20 gal of water. When the Thermostat is satisfied, the cycle starts like any other TL machine. The Solid tub machines operate differently, you put the clothes and detergent in the tub, the machine fills and heats into the outer tub, before pumping the water into the wash tub and the cycle starting.
The self heat machines here, all run on a standard Powerpoint, which means they're limited to 2400watts, with a heating time of 90 minutes that's 3.6kw/h of power used, so you can see why suds savers were so popular for such a long time.