Achieving long exposure effects with a variable Neutral Density (ND) filter allows photographers to blur motion (water, clouds, crowds) in bright daylight by reducing the amount of light entering the lens, offering a flexible, all-in-one solution. A variable ND filter typically provides 1 to 9+ stops of light reduction, allowing for long exposures without carrying multiple fixed filters.
This video shows how a variable ND filter can be used to control light and create long exposure effects: