The answer is everyone! Here's a few scenarios of what we mean. If you live in town and you have municipal treated water you still run the risk of contamination entering your home especially when you see Boil Water Advisories and broken water lines in your homes area. When you live in rural property and have pre-treatment you may think that UV isn't important but the best way to describe it would be to say its like an insurance on if your pre-treatment stops working the way it should the ultraviolet will still disinfect the water from the things not seen with the naked eye.