It was not used for grain threshing.
On the farm a stone device could not avoid being broken by the jostling of being dropped and banged into by animals. Not to mention the wear from sand in the grain and loose stones caught between the blades.
The schist may have derived energy from spinning in a moving fluid other than water, otherwise it would have been made of wood.
Maybe boiling honey? ( Seriously, the Egyptians could have heated honey and used it in a temple for a turbine apparatus that operated the pulleys that opened temple doors for special times of the year. Honey was sacred, and the scent may have stood in for the presence of a holy entity.) Or perhaps heated sandalwood oil, for the same reason.