Probably not in your bath, but in houses with water tanks at least one of the cold taps will be fed directly from the mains supply. If you are drinking it then you want it straight from the mains rather than from the tank in your loft that’s been sat there for a while and had all sorts of things falling into it

but in houses with water tanks

This is another big surprise to Americans, which went from no running water to high-pressure mains.