It's necessary to lower the freezing temperature of the mixture in order to get the fine crystal structure of frozen sherbet.
Ice surrounds the mixture in an outside container.
As the ice begins to melt, you add rock salt to it, which dissolves in the water and lowers the freezing point of the remaining ice. And you'll need this lower freezing point to turn the mixture in the center into a frozen sherbet. The heat energy from the temperature of the sherbet mixture, and the temperature of the room. Turning the crank on the machine keeps this heat energy evenly distributed throughout the sherbet mixture. This heat melts more ice, which dissolves more rock salt, which continues to lower the freezing point.