The only household uses you could use a lower grade of tap water for would be toilets and clothes washing (and then only if you run the clothes on a hot cycle).
https://www.strongtowns.org/journal/2016/2/7/chipping-flint
Hong Kong, for example, has separate water infrastructure for salt water, which is used for flushing toilets and other non-drinking uses.
What is it used for? Cooking?
It's surprisingly easy and quick to run way more water than you could drink in a day just for some daily cleaning duties.
Come to think of it, I drink around 2 liters per day; that's six mugs of coffee plus maybe a couple glasses of water.
How much water do I spend when I flush the toilet once and then wash my hands? I do that quite a few times every day.
I'm not sure about the water used for cleaning though. If you want your hands to be clean before eating, why would you clean them in something your can't safely drink?
Water usage is twice as high in summer as it is in winter. There's a reason why, when mandatory water restrictions go in effect, the first thing that's banned is watering lawns.
https://www.statista.com/statistics/495373/household-water-c...
http://www.hamiltonproject.org/charts/domestic_water_use_per...