ND filters?

The thing about ND filters is that it allows you to control how much light is coming into your camera so you can pick the aperture that creates the depth of field to best capture the photo. This way you have even more control over how the photo looks, instead of letting the amount of light dictate your DOF.

I should also add that ND filters do not effect the look of the photo. ND filters are like putting sun glasses on.

Nd filters are used during video when the lighting is to bright to retain a wide f-stop. Makes thw background blurred and yes it really makes a film look during daylight. Theyre also used for lobg exposure pictures during daylight.

Necessary? No. But, if you want any sort of "film-look" or small amounts of motion blur (vs choppy very sharp looking motion) you need to run a shutter 1/180 or lower. You can only stop down your aperture so much too, past about f16 the image becomes less sharp. Sure, you could go iso 100 f22 and 1/500-1/1000 shutter and you'll be correctly exposed on a sunny day, but with a 1.2ND (4 stops of light, 1/16) you could run a shutter of 1/125 aperture f8 and it'd may look better to some people.

I use a 1.2ND for any ski stuff in the sun, I like shooting 1/125 or 1/180 for 60fps and like keeping my aperture in the "ideal range" of f5.6-f11.

it depends on what your shooting or how you want it to look really. NDs help you acheive a shallow depth of field when filming because you are able to open the aperture more. they can also be benneficial just for the fact that most lenses are sharpest anywhere from f5.6 to f8 or so.  if you are out shooting on snow at 30fps or 24fps, even with a really low iso, you still will need an aperture of >10 with a normal shutter instead of the lenses sweet spot

i use my ND all the time and think it helps a bunch.  even if you have a bright indoor scene for a narrative, you can use an ND to make sure you get the right "look" you are going for and you don't have such a closed aperture that everything in the frame is in focus.