ND filters are often used when shooting video because shutter speeds need to be kept low in order for the video to have a fluid “filmic” look to it. A video with a shutter speed of 1/1000 for example, looks jittery and weird. The general rule with video is to use a shutter speed that is twice what your frame rate is. If you are shooting 29.97 fps video, you would use a shutter speed of 1/60 second.