Cryptosporidium and giardia cysts can survive chlorination and cause fatal illness in persons who have weakened immune systems. This is not theory. In 1993 more than 400,000 people developed gastrointestinal illness when a water treatment plant in Milwaukee, Wisconsin failed to control turbidity at a critical time when the water source was rich with cryptosporidium cysts. The list of concerns over drinking water goes on. Metals can contaminate people's water from both the source and the distribution system.
Contamination can enter at any point, all the way down to the pipes in a person's very own home.
Once water leaves the treatment plant it can travel many miles, and move through various storage systems and pipes before reaching your cup and pot. Along the way it can pick up lead, mercury, and other metals. Water at the tap can have higher levels of unwanted and unhealthy metals than the same water inspected at the treatment plant. Two of many examples being the herbicide atrazine and the pesticide lindane. Public water system treatments are generally ineffective for these VOC chemicals.