There are two main reasons. One, after we create it, we don’t want it hanging around. We want it removed. After it is removed, it must go somewhere and be turned into something safe that will not hurt people or the environment.
Two, water is becoming a more limited and expensive resource, especially in highly populated areas that are prone to drought. There just isn’t enough to meet the demands. Reclaiming and reusing water is gaining more and more focus as a critical environmental and sustainability issue.