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I use filters less and less, instead relying on blending for my landscapes. For times with truly harsh light such as in snow and desert sand shots, a circular polarizer is helpful.
A CPL can also be useful in controlling water (and window, metal, etc.) reflections. I'm not saying to use it to eliminate those reflections, but to control what highlights you want in your shot.
I have seen photos where CPLs / polarized light were used to eliminate all specular highlights which lead to dead looking shots.