An ND filter can help with creative exposures by reducing the light into the sensor. For Example a 10 stop Neutral Density Filter such as the B+W 3.0 or the Lee 'Big Stopper' would reduce the light reaching the sensor by10 stops... that is a 1/50 second exposure would be extended to 20 seconds or 10x longer.
Uses: A 20 second exposure could be used to blur waves into a soft, misty effect as illustrated in the 20 second exposure of a Californian beach shown here.
I have the B+W 3.0 10 Stop ND Filter but if you have already invested in the Lee filter system the 'Big Stopper' filter by Lee is worth a look. It has a slightly more natural color cast than the B+W which does give a warm tone, although a lot of people find the B+W tone pleasing in most circumstances
It is very hard to focus or compose a scene through a 10 Stop filter unless the scene is very bright to begin with and an advantage of the Lee Big Stopper system is that it is easier to remove the filter for focusing and composition. The Round screw in B+W filter takes more effort to add and remove from the lens.