The whole purpose of a water treatment plant is to remove materials and chemicals that are harmful to humans and reintroduce the water into the ecosystem, ideally for human consumption and use.
Therefore, using the correct type, amount and balance of chemicals in such plants is essential in order to successfully treat the water and/or to deal with specific substances found in a particular water supply. It is also important to remember that whatever chemicals are introduced, they are also going to re-enter the water supply, so using the correct amount of treatment chemicals is key to ensuring that the water is still safe, otherwise, you are treating one kind of contaminant only to introduce another.
Lastly, these chemicals getting into the water supply also means they get into the habitat and ecosystem of other species. Too much or too many chemicals can, for example, affect the health of bird and fish species.