From Ange's current level of understanding, holy water is actually not very pure. The purification spell merely kills off bacteria and similar entities within the water, but it doesn't eliminate their remnants.
Nevertheless, such water is already considered incomparably clean given the current level of production.
Can you imagine the situation of contracting diseases from leech-infested water, from washing private parts, from drinking unclean water, or from food poisoning? In this era, such cases are everywhere. Water is not just the source of life, but also the source of diseases.
The purification spell can't remove impurities in the water, yet it can cleanse bacteria, parasites, and such within the water making it incomparably cleaner than natural sources of water.
The difficulty of purification magic is not high, even a devout believer can cast it. Many diseases don't require special medicine, just clean water is enough.
