If you shoot photo (or specifically video) outdoors in the sunlight, you NEED a variable ND filter.

For video, if you're shooting 30 FPS, you need to keep your shutter speed at 1/60 (because of the 180 rule) for natural blur and smoothness. If you want a nice blurry background at f/2.8, even at ISO 100, there's a great chance your image will be horribly exposed. Your only options are to increase the shutter speed (and compromise the natural look with distracting "choppiness") or crank up to f/16, which will flatten your image and get rid of that blurry background you wanted!

In comes the variable ND filter. When applied, like variable sunglasses, it allows how much light passes through, kind of like an intimidating bouncer at a night club. So just twist the ring on the filter and you can achieve your shot perfectly!

With this specific filter, two small things bummed me out:
1. The ring doesn't have actual stops, just continuous rotation so you can't really keep track of the specific amount of light you're letting in.
2. When the filter is on my Tamron 28-75 2.8 lens, I can't put the lens cap on at the same time. Not a deal breaker, but annoying when you want to quickly take a picture or shoot some video.

But for the price, it's hard to argue this isn't a great purchase.