It seems computers can do most anything with an image now a days. So why bother with all those cumbersome filters? The answer is that most of them are still superior to the techniques used with a computer. Plus, if the image is more accurate coming out of the camera, less time and effort will be needed in post processing. Lastly, understanding the play of light and filtration is an important cornerstone to understanding photography, which is simply put, capturing light.
This post will concentrate on the polarizing filter and more specifically, the circular polarizing filter. So what is a polarizing filter? DPS already has a great post on How to Use and Buy a Polarizing Filters which should be your first stop in understanding these filters. In a nut shell, a polarizing filter will reduce the reflected glare off of water, sky (particulate), metallic surfaces and anything else with a good reflective surface, even including leaves. It helps increase contrast and provide better, truer color when used properly.