Polarising filters provide colour and contrast enhancement.
Reflected light often shows up as whitish glare that washes out colour in an image.
A Polariser corrects this problem producing deep, dramatically blue skies.
It also removes glare from non-metallic surfaces, such as windows and water.
Colour saturation in general, especially outdoors, can be improved significantly.
A Circular Polariser should be used on cameras with beam splitting metering systems commonly found on auto focus SLR's.