Water quality is important because it directly affects the health of the people, animals and plants that drink or otherwise utilize the water. When water quality is compromised, its usage puts users at risk of developing health complications. The environment also suffers when the quality of water is low.
The availability of high-quality water is a key determinant for human, animal and plant survival. Without water, living things could not survive, making water quality one of the most important factors in whether anything can inhabit an area. Though water for human consumption is usually treated, there's always the risk of accessing water that has not been treated, which is why the water used in an area must be protected at the source.
As water moves from one stream to another, any contamination in the water flows along with it and has the potential to reach thousands of communities. Water quality is also important in determining plant and animal health. In many farms, water from nearby streams is used for irrigation. If that water is contaminated, any food grown with it or animals that graze on the grass have risk of contamination, as do any people who eat those crops or animals.