Why is it important to remove nitrogen and phosphorus from raw sewage?

Nitrogen and phosphorus leaked into the water systems increase nutrients available for algae, leading to eutrophication. Because of the amount of algae growing in water systems, bacteria use up a huge amount of oxygen during metabolism in order to clear away the algae. As a result there is much less oxygen available for other living things, and thus their oxygen requirement can not be met, and consequently there is a mass death of these organisms.