Why have an off position for the heating dial on electric showers?
Note: this answer is based on my experiance in the UK, I do not know if this are the same in the rest of the world. 

Traditionally the "temperature" control on an electic shower did not directly control temperature, instead it controlled flow rate. For a given input water temperature and heating power, a lower flow rate resulted in a higher temperature.

Nowadays nearly all electric showers have thermostats to prevent the output water becoming unexpected hot in limited water scenarios, but the control scheme in normal operation (not limited by input water) remains the same. You set a power control to select how much heating power (if any) you want, then use the main dial to control the trade-off between temperature and flow, or in the case of "cold" mode to control the flow.