Chlorine is used in water purification because it is effective and it is persistent.
Two very good reasons for removing it from water before human use. The action of chlorine is to oxidize compounds with which it comes in contact. These compounds can be bacteria, the stains in our clothes, Vitamins C and E, thyroid hormone and many other compounds important to human life. The very reasons that chlorine is used as a water purification agent are the reasons we should ensure that it is no longer in the water when we finally use it. It is indiscriminate in its action.