nd grad filters, as it name suggest, is graduated, ie one side is dark and the other clear. the place where the two meet is the line of transition. this line can be hard (clear distinct transition) or soft (no clear line, the filter gradually darkens over a range). When you use a nd grad, you are trying to control the contrast. unless you compose with your line of contrast smack in the centre of the frame all the time, a screw in nd grad filter isn't very usefull at all as the line of transition is normally in the middle of the filter. Slot in filters on the other hand, allows you to position that line where ever you like, and you can compose better.