Why Do We Need To Take Shower Before Swimming?

In a public pool situation, it's a health issue (everyone just feels better about it) but in reality, chlorine (the disinfectant used in the pool) becomes less effective with more bacteria--in other words, the more bacteria, the more the chlorine "demand" will be and thus the more chlorine needed, and in turn more expense, not to mention the constant "checking" of the chlorine residual in order to keep it up with health department standards.

The same reason you wipe your feet on the mat before entering your carpeted house. Its not about the shoes, its about the carpet.

With a swimming pool, it's not about washing you, it's about keeping the pool clean. Some pool filters are rendered less effective by sweat, body oil, suntan lotion, and other such things that people have on themselves.