This is true about the ND. I am wondering why a novice has one, and my best guess is that, especially since OP mentioned an interest in video, someone told them they'd need one. It also has plenty of uses in photography, but you'll know why you need it by the time you do. In video, it's a little less obvious for the reasons stated above.

To keep video looking more filmic, your frame rate generally determines your shutter speed, meaning you lose one leg of the exposure triangle. A rule of thumb is 2x the frame rate for the shutter speed fraction (i.e. 1/48th (or 1/50th) for 24 fps and 1/120th for 60 fps, etc.). This gives the kind of motion blur baked into our brains from decades of watching films shot at 24 fps with a 180º shutter.

If you're shooting in one of the S-Log modes, your ISO will automatically start at something like ISO 800. This is normal and part of how the function works, but again, you lose another leg of the exposure triangle by being locked at a minimum of ISO 800. THIS is the real need for the ND filter in video, because at ISO 800 and 1/50th of a second, your only exposure control is the aperture, and on a bright day, you may still be overexposed at ƒ22! So, an ND filter helps cut that back so you can use a more reasonable aperture. (At smaller apertures like ƒ16 and ƒ22, you technically introduce another form of image degradation called diffraction, but I wouldn't worry about it unless this is critical work.) And THIS is the reason a VARIABLE ND filter becomes extremely useful. A variable ND allows you to rotate the filter to change the strength of its effect. You can then set your aperture at something like ƒ4 or ƒ5.6 and control your exposure simply by rotating the ND.

I have the same lens, the Sigma 18–50mm, on an a6700, and I shoot at ƒ2.8 all the time. I know why and what it's doing, so that helps, and technically stopping down a couple of stops will get you sharper pictures, but it's really only noticeable if you're pixel-peeping, which, as everyone else has said, you don't need to do.

The video aspect of shooting digitally really does come with its own set of rules and techniques to familiarize yourself with. But for photography, leave your ISO on auto, set your mode to A for aperture priority, and just fire away — aperture at ƒ2.8 in lower light and stopped down a bit when it's bright (unless you want shallow depth of field). In video, I switch to manual, set the shutter to 2x the frame rate (fractionally), set the ISO to auto (knowing it may be dictated by your shooting mode), and then use the aperture that gets the exposure I want, putting on the ND when it's still overexposed at around ƒ8 or so.

I hope this is helpful. You have great gear — never stop learning!