Polarizing filters block scattered light, increasing contrast in skies and removing glare in water and on foliage. Out of all of my filters, this is the one that almost always ends up in my bag. Some people even keep a polarizer on their lens full-time. I would caution against that idea, because polarizers not only affect image contrast, but they also reduce the amount of light coming into your camera. Because polarizers are dark, they require either slower shutter speeds or higher ISOs to get a correct exposure.
Polarizers are great for when you want to add contrast to a sky, but their effect is not uniform; the maximum effect from a polarizer is when you’re pointed 90° away from the sun. When you use a polarizer with a wide-angle lens, you can end up with uneven skies. Moreover, you can add contrast to skies with post-processing software, such as the Polarizing Filter effect in Color Efex Pro 4.
The one effect you cannot replicate in post is glare removal. If you’re shooting landscapes of water or forests/foliage, a polarizing filter is the only answer for removing glare from reflected light
Verdict: The polarizer is one filter that should be in your bag, but it’s more for removing glare than for increasing contrast, especially with wide-angle lenses.