Sufficient clean water is vital of importance to humans, animals and for the environment. No one wants to live near polluted, fetid water in ponds or canals. In the summer, we want to be able to swim in open water without worrying about getting sick from, for example, blue-green algae or botulism.
It is easier - and therefore cheaper - to produce drinking water from high-quality water. Good water quality is just as important to plants and animals. It makes for a larger variety in flora and fauna, and everyone enjoys seeing fish swim in the water. In short: clean water benefits everyone.