I constantly see those projects that would benefit from a screenshot in a readme, but for some reason the thought about adding one doesn't even come to the authors' mind, even though it's pretty obvious. I'd really like to know what's going on.
Case in point, I recently wrote a CLI to show which of your AWS infra is not captured in terraform, and you can either print the result as a json to consume by a machine or generate a dashboard with some charts. It took me an embarrassing while to realise that I should have included a damned screenshot in the README, in fact I think I only realised when I wanted to show it to my brother.
Also updating them to match every change in the code is a pane. Most app stores have ways to track screen shot assets along with releases, and even GH has a way to add image assets to release notes without tracking the images in Git, but not very many projects realize that's an option.
Which is a bummer because it is a complete different point of view from the more typical user experience
In many cases the repository isn't meant to be more than for source code too, not everyone use their git repository as also the marketing page, that'd go somewhere else.
With that said, grapheneos.org doesn't seem to have any screenshots either, which I also don't understand and think is a bummer. Even though the point of the differences with GrapheneOS might not be mainly visual, just showing what it looks like seem like a no-brainer.
It's like we've had this ancient knowledge passed down for generations but people constantly just ignore it.