Locus Studio

CPL filters are used to increase contrast and saturation for clear and partly cloudy skies (it can darken some regions of the sky - the polarization band). They are also used to remove glare/reflections from vegetation/water/non-metallic surfaces which can increase color saturation of the scene. To some degree, a CPL will also help to cut through atmospheric haze in distant landscapes. A CPL isn't going to be of much use on an overcast day when the light is soft, there's no blue sky, and there's no glare on the various surfaces in the scene.