At some point or another, most photographers have wanted the look of a long exposure, such as moving clouds, blurred motion, creamy water, but are limited by the reality of exposure.  If we take a typical sunny day, we know that our exposure at f/16 and ISO 100 will be about 1/125 of a second.  No smooth blur there.  The solution is a Neutral Density filter.  And, if you want a big drop in light transmission, the go to answer is a Lee Big Stopper.