Water treatment plants in the US are required to have a minimum residual disinfectant content at the farthest tap from the plant. This helps with any contamination that might occur due to a broken water line. Ozone doesn't provide the residual so some chlorination is applied after treatment if O3 is used. ClO2 & O3 are expensive compared to bubbling Chlorine through the water even though they are more effective as disinfectants.