Screenshots are the wrong tool for this. You cannot scrub backwards quickly, you miss the frame you wanted every time, and you end up with a folder of near-identical phone screenshots instead of a clean set. This grabber walks the clip for you and lays the results out where you can see them all at once.
How the interval works
You set one number: the gap between frames, in seconds. The default is every 2 seconds, which is a sensible starting point because it gives you roughly one still per two seconds of footage. From there it is a trade in both directions. Halve the interval and you double the number of images, which means a finer chance of catching the exact moment you need but a much longer list to pick through. Push it to 5 or 10 seconds and the result becomes a quick visual summary of the whole clip.
What makes a good frame grab
The quality ceiling is the video itself. Frames are pulled at the source resolution, so a high-definition clip gives you high-definition images, but nothing can restore detail that was never encoded. A heavily compressed source will produce stills that inherit its artefacts, which is usually visible as blockiness in flat areas like skies.
Motion blur is the other thing to watch for. A frame grabbed mid-pan will be soft no matter how good the source is, which is exactly why a tighter interval is worth using when the shot moves.
Picking the keeper
The results appear together so you can compare them side by side rather than scrubbing back and forth. Clicking any one of them downloads that frame as an image, and you can repeat for as many as you want. Most people end up taking three or four out of thirty.
What people do with them
Stills from a clip are useful for thumbnails, for contact sheets, for pulling a slide or a chart out of a screen recording, and for checking whether a shot actually landed. If a frame is on screen only briefly, grabbing it at intervals is a fast way to find the moment, then trimming that stretch of video down so the frame itself gets more screen time.
Using frames to review a clip quickly
There is a second use for this that is easy to overlook. Stepping through a video at fixed intervals produces a rough contact sheet of everything inside it, which is a fast way to review footage without watching it twice. When you are checking a long recording for the one moment that matters, or looking for where a particular shot appears, the frames give you a map of the clip and you can jump straight to the interesting second instead of scrubbing back and forth. The same frames make a reasonable thumbnail source too, since a still pulled from your own footage says far more than a generic stock image, and picking one is a single click.
On choosing an interval: two seconds suits interviews, walkthroughs and anything where the pictures change slowly. Anything with fast action wants one second or less, because a shorter gap is the only way to land on a clean pose rather than a blur.