Variable neutral density filters enable you to choose exactly how much light you want to restrict for your camera's sensor, allowing you to capture long exposures for effects that freeze movement, create light trails, or make moving crowds vanish.
However, even the best variable NDs aren't perfect. Their density range is somewhat limited, and if you get close to their maximum density, an obvious X shape becomes visible. Of course you could switch to separate neutral density filters to avoid this, but that means more gear to carry.
