To any Landscape photographer, ND filters and polarizers are useful tools for great photography. Here I will be talking about the differences between constant ND filters and variable ND filters and how to use a variable ND filter. A ND filter is neutral so it should not affect colour or saturation but only reduce the amount of light.
A variable ND uses polarizing to vary the density hence you get a similar effect as a using a plain polarizer but using a wider range of polarizing effect.
Variable ND filters do use polarizing glass. These are useful in certain situations. You may own an 85mm f/1.9 legacy lens that does a great job in normal conditions. But if you happen to use that lens on a very bright sunny afternoon at the widest aperture then this lens is difficult to use in those conditions because of the blown highlights it delivers. Blown highlights are overexposed areas of the image.