I use one when I fly over water. It reduces the glare and if the water is smooth you can see what’s under the water better. People use polarized lenses for fishing for the same reason. So basically it’s glare reduction.
To expand a little on what kabonski said, polarizer filters are designed to remove certain types of light from your shots. This includes glare, lens flares, and most other kinds of reflected light, all referred to with the blanket term "polarized light". As a side effect of the removal of this light, you will also typically see a boost to contrast and color saturation in the image. The most popular use of this type of filter for drone flight is to cut out the glare from the surface of the water, but polarizer filters are also beneficial in environments that are snowy, have a lot of windows, or on days where the sky is brighter than usual.
ND filters, in contrast, reduce all of the light entering the camera by a certain amount depending on the strength of the filter. They are more akin to sunglasses for the camera, and have no effect on color or contrast. For drone flight, they are most popular for folks wanting to use the popular 2:1 shutter speed/framerate setup in order to give their shots a nice, cinematic motion blur.
Both types of filters are useful tools to have available when shooting due to the additional lighting control they offer, especially for anyone looking to capture the highest-quality content possible. If anyone would like any additional information about the way the filters work, please don't hesitate to message us here or send an email to [email protected]. Hope this helps!