Variable neutral density filters use two counter-rotating polarising filters to create an adjustable darkening effect. They have limited usefulness for stills photography, where regular fixed-effect ND filters are more useful and tend to give better results. Regular ND filters can also give stronger darken for longer exposures and blurred water and cloud effects.
However, variable ND filters are useful in video because here it’s often important to maintain the same aperture (iris) setting and shutter speed (or ‘shutter angle’) to keep a consistent visual effect even in changing lighting – you can adapt to different light levels by adjusting the ND effect.