Why have an off position for the heating dial on electric showers?

So – some background. I'm fairly new to the concept of electric showers – showers that heat cold water themselves with internal heating elements. With my shower, I have two dials – one for temperature, and another for adjusting the "Eco" setting. This eco dial has three positions – I'm totally blind, so not sure how these are physically labeled. During testing, it appears that if I have my temperature setting on the highest value and do the same for the eco dial, the water is at its hottest. If I then keep the temperature at the same setting but turn the eco dial down, it appears that the shower will wait until the water cools down before it will then heat some more – and on its lowest setting (which I assume is off or similar), the water doesn't appear to get heated at all.

So – my question. If no heating is performed on water when the heating/eco dial is at its lowest value despite the temperature dial being on the highest setting, why have an "off" value for this dial? Is this effectively the same as simply turning the temperature dial down to its lowest value?