Want to blur a waterfall with a 4-second exposure, but all you can get is 1s at f/16? Either stop down to f/44 if you can and blur the whole lot with diffraction, or add a 3-stop ND.
Want to shoot wide open at f/1.4 at a max of 1/1000s, but the slowest film you've got is ei50 and sunny-16 says you need ei8? Add a 3-stop ND and you're at a sensible speed.
Want to shoot a building where people keep walking past? Add an ND to get to a few-minutes exposure and the people magically disappear.
Want to shoot the sun? (I've done it, it's fun). Add an ND10,000...