To my understanding they are mainly for video.  I watched a few YouTube videos a few years ago showing the difference and what a person really like to watch.  I was surprised that with video you do not always want the sharpest image on movement since the eye/brain can not or does not want to see all that detail.  Adding a neutral density filter will change the opening of the lens which changes how "sharp" the camera may record.
This is a very basic description but go to YouTube - there is a lot of videos explaining and showing the difference.