ND filters reduce the amount of light entering the camera without changing color. ND filters are particular useful in aerial photography since most of the time the subjects involve bright sunny skyscape or landscape scenary.
ND filters are essential for reducing the rolling shutter effects in video photography. A very bright subject will force the Phantom's or Inspire1 ZenmuseX3 camera to choose a very fast shutter speed being with a fixed (2.8) f-stop lens. Since the CMOS sensor used in the Phantom4/3Pro/3Advance/ZenmuseX3 camera records from top to bottom, a quick pan or an extremely fast moving subject can render the video to appear to be skewed, distorted or choppy when capturing with high shutter speed.
ND filters can help the camera to shoot at a desirable slower shutter speeds in video mode, gives the video a cinematic effect and the resulting natural motion blur can help mask the rolling shutter effects.
The other use of ND filters is to increase the exposure time to achieve a wide variety of artistic effects in still photography. It is particular effective in softening the appearance of moving water or moving clouds, blurring waves of blowing grass or even blur moving people in an urban settings.
ND filters are one of the simplest filters to use and their effect cannot be duplicated easily with post production digital enhancements.