Limiting depth of field is a very useful way to emphasize specific elements of a photograph and for that reason it's almost always used for portraits.
It also compensates for portrait photos being best taken from a distance using a telephoto lens. The reason those are best is because it captures the face people remember instead of the face they actually see. The reason compensation is needed is because the same lens configuration will get a much smaller depth-of-field up close but a much higher one at a distance.
Difference being, the face is the content, the background is noise. Here, the screenshot is the content. Ex. The only physical feature of my face i like is my eyes. Yet, it'd be weird if I depth of field'd everything but my eyes.
I'm sure this has uses but it's hard to argue it does from fundamentals of photography
There are lots of cases, especially in say marketing images, where the entire screenshot is not the content. You may want to highlight a specific bit of UI while still keeping the general background context instead of cropping into a small image.
This lets you click a portion of the screenshot to bring into focus.
At the end of the day it's a fun toy web app, but I don't think the general concept is useless.
I hadn't actually seen the demo when I commented! I'd assumed it was using an AI model to estimate distance from camera of the elements of a photograph and then reproducing a shallower depth-of-field. So my comment isn't all that relevant.
Depth of Field being visible is an artifact of 2D screens & prints. Our eyes do have depth of field, but when we look straight at something they very quickly refocus onto that thing, so it's often very hard to perceive. With a static photograph or picture on a screen we can't refocus on what we're looking at within the picture automatically.
It's not that our eyes don't have depth of field, it's that they operate differently than fixed photos or pre-set DoF effects in a game.
Blur is completely natural to human vision, to any kind of vision. Look far away while bringing your hand close. Blur. Now look at your hand. Everything else goes blurry. You can even unfocus your eyes while looking at something. Now everything's blurry.
This is unnecessarily snarky. I would hate to use one of these depth-of-field screenshots for, say, an attachment to a bug report.
But it looks like a great visual effect to use on a marketing site, especially to highlight a specific part of a screenshot to get across whatever you want to emphasize.
