Neutral Density filters cut donw light, like sun glasses.
If you have a very fast lens... like F/2.8, you might want to use 2.8 in bright sunlight, like if u want to have the depth of feild effect.
Mostly people use them to prolong exposures...
so u get like 1 second exposures (use with tripod) in the day for example. but why?
some use it to shoot pictures with water, to make flowing water smooth and velvety or shoot a busy road and have the effect of cars fading away and people look like they have been smeered on to ur pic, a very interesting effect.