Circular Polarizer, when used properly, can help do something in your image in camera that you practically cannot duplicate in post, which is eliminate reflection in water to see what’s under the water. This is big for landscape shooters who wants to showcase the rocks and details underwater.
Eric Leslie explains in greater details on the usage of the circular polarizer and what he uses it for and what he doesn’t use it for. I do agree with him in not using it for darkening the sky because you really have to align yourself 90 degrees from the sun to get that effect or else you get an uneven darkening of the sky. Better to use a graduated ND filter or do it all in post off from either one RAW or a couple bracketed RAW files so you’re not limited in your composition.
The other benefit of the circular polarizer, he says is that you can bring back the contrast and saturation that is lost when shooting near any reflective surface, be it water or rocks. Take a look at the difference here from his example.
There are several other usages that Eric goes into, so be sure to read his article in detail on How to Use a Circular Polarizer
You can see more of Eric’s work at his website, EricLeslie.com