When there is a burst in the house's potable water system and the stop cock has to be shut OFF a tank of stored non potable water is useful while waiting for the plumber.
Quoting from
HERE
We are legally required to supply a minimum water pressure of 7 meters head (0.7 bar) to your boundary stop tap. Pressure is measured in bars and 1 bar will push water to a height of 10 metres to your home.
How does this relate to a system that requires more than 1 bar pressure to operate correctly ?
Until the water company installed a booster pump my cousin's water supply at the kitchen tap for drinking could be very slow. ( she kept a supply of bottled water for the times when the trickle was not enough) The cold water store in the attic would refill slowly over night and supply the non-potable needs of the house during the day.
Her house was one of the highest on cliffs overlooking the sea and holiday makers day time water requirements were reducing pressures in the mains such that the pressure at her house was well below 0.7 bar.