Truma water heater

Paul Rainbow

I am a bit confused about why I need a separate Truma storage heater in my 'van as I always get hot water from my taps even when it is off. 

I must have a water heater as I also turn the water heater on from the control Panel. Why do I want to turn of the Truma heater as well?

Does it do something else I am unaware of? Or maybe only works on gas, whereas the water heater may oBly work off mains? Just guessing here, any advice would be welcomed!