They're pretty essential for any situation where you can't just stop down the exposure in the camera. If you want a long exposure shot of some crashing waves on a bright day for instance, you might not be able to just increase the shutter speed or narrow the aperture without ruining the intended photo.
I personally like to make dreamy photos wide open with relatively long shutter to accentuate movement. Some of it in full sunlight. Yes, ND filter is essential then.
