CP (circular polarizer) filters are being used for several reasons. They help to improve contrast and get more vivid colors (interesting for landscape for instance) and they also kill or reduce annoying reflections (for instance in water, a window, etc...). I use a CP filter for some landscape shots and when I am traveling and I want to take pictures from a car or a bus, through the windshield or a window. If I would not use a CP, I would take a picture of the reflections in the window.
Without the filter: ("bad" reflection and low contrast) - shot from a car
With the filter: difference should be obvious
