You can read thia article on Wikipedia to learn why we need cooling towers: http://en.wikipedia.org/wiki/Heat_engine

Basically you can only get electricity out of a pwoer station by having two different temperatures and the greater the difference the more efficient your power generation will be. You have some sort of heat source, be it coal, gas or nuclear and you need a "cool source" as well which is provided by water circulated through the cooling towers.