There are times when you just want to reduce the amount of light coming into the camera, to get a specific effect, such as slowing down the shutter speed.
In other words, you've already reduced the ISO as far as it will go, and your shutter speed is still higher than you would like, the only way to reduce the light further, is with an ND filter.
Slower shutter speeds do two things. They reduce the jello effect and they give motion blur to your video, for a more realistic cinematic effect.
In cameras with adjustable apertures, less light gives you less depth of field, another useful effect, although this is not available on the Mavic camera.