The Lee 150 x 150mm 0.9 Neutral Density Filter is for the SW150 Filter Holder.
A Neutral Density filter permits you to create images that would ordinarily not be possible without such filtration. For the landscape and nature photographer, using this filter allows you to create a "blur" in a rippling stream rather than freezing the motion which is what most photos traditionally display. Even in big city, you can shoot traffic and create a sense of motion. Or, if you prefer nature photography, catch a bird in flight and watch the trail behind him as he flies.
ND filters are valuable, for both film and digital, because they allow you to use slower shutter speeds with fast films and ISOs when shooting under bright light - a beach, the desert, and other such scenarios. The trick with neutral density filtration is knowing which grade to use in order to achieve the results you want.
Determining which neutral density filter yields ideal results for any given lighting situation takes knowledge, experience and a collection of such filters. Choose the filter strength which adjusts the lighting to stay within the exposure latitude (greatest difference between bright/dark values) which still shows details in both of the digital or film medium in use.