Every now and again I get asked about filters for portraits. I don't use them often. In fact, my general rule for filters is I don't use them unless I'm planning for extreme conditions, or the filter allows me to do something in camera that I'm not currently equipped to manage in post-processing.
That said, I do tote around a 3-stop X3 Breakthrough neutral density filter and a 6-stop X4 Breakthrough neutral density filter in my gig bag all the time. A neutral density filter reduces the amount of light coming through the lens much like sunglasses. This is not a particularly useful tool when using ambient/continuous light sources because they effectively slow down the shutter speed, potentially resulting in softer or blurrier images; but when used with flashes and strobes, neutral density filters allow photographers the flexibility in manipulating the aperture and thereby the depth of field (in focus/out of focus range) of an image.
When using a flash or a strobe, most modern digital cameras have a maximum shutter speed (also known as sync speed) they're capable of achieving. This generally falls between 1/125s and 1/320s depending on the camera and sensor size. This is because shutter mechanisms use a two-curtain system to expose the sensor. Picture a stage with two curtains from above. One is currently drapped, obscuring the stage. The second one is rolled up at the top. When the director says "action" the obscuring curtain falls to the floor so the audience can now see the stage. When the director yells "cut" the second rolled up curtain drapes to obscure the stage once again. The amount of time between the "action" and "cut" is the shutter speed. In order to simulate shutter speeds faster than the sync speed, cameras drop the "cut" curtain even before the "action" curtain hits the floor. This isn't a problem for continuous light because each part of the sensor sees the light at some point before the exposure is recorded. This is a problem for a flash though, because the burst of the flash gets obscured by the "cut" curtain falling before the flash fires blocking a portion of the stage from the flash's light. Because this mechanic limits a photographer's shutter speed when using flash, photographers often have to compromise on their aperture to reduce the amount of light coming into the camera making for portraits that have less foreground/background blur separation.
The good news is there are several workarounds to this issue. One solution is to use a flash or strobe that has a high speed sync or hyper sync feature. As a Fujifilm X-T2 photographer, there aren't many off-camera high speed sync or hyper sync flash solutions available. Another solution is to purchase a camera with a mechanical shutter mechanism. The X100 series does have a leaf mechanical shutter, but it does not have the flexibility of an interchangeable lens system. A third solution is to use continuous lights. Continuous lights often don't have the power output I need to get the exposure I'm after, or if they do, they're incredibly hot (like a stove) and difficult to use with light modifiers. They're also difficult to power on location. For these reasons, I opt to use neutral density filters.
In the above photo, the sun is setting to the camera's right, lighting the model's hair. I used a flash with the Magmod maggrip, a ½ color temperature orange (CTO) gel in a maggel sleeve, and the magsphere on a light stand as the main light to the camera's left. With my shutter speed set to 1/250s, my aperture needed to be set in the f/8 range to get the exposure I wanted, but I really wanted an f/1.4 depth of field. To do this, I put the 6-stop neutral density filter on the lens, opened the aperture 5-stops to get it to 1.4, and reduced my shutter speed to 1/125s to get that 6th stop accounted for. And that, in an unusually wordy blog post, is how and why I use neutral density filters on occasion for portrait photography.