Please excuse a dumb question from a beginner. I have used ND filters in still photography for slowing time exposures for waterfalls etc, but what is the advantage for the Mavik? Is it more about depth of field than exposure time? Are there some good with/without examples?

I'm a beginner myself.  The rule that I've heard is you want your exposure to be double your FPS.  I shoot 30 FPS, so I try and get my exposure to 1/60th.  ND filters help me do that.  I notice weird flickering when I film without them during the day.