What does a screenshot of a game tell you about the engine or the developer experience? If you can push triangles and run shaders you can render anything.
A screenshot (better yet, a GIF) gives you at least a basic idea of what the engine's renderer can do.
You can push triangles and run shaders in pretty much every UI framework on earth and yet for some strange inexplicable reason people tend to want to see what the framework can actually do.
For a start, it tells you the engine can actually be used to make a full finished game — which with hobby game engines isn’t a guarantee. If you want me to use an engine, I’d like at least one finished game, preferably even released on Steam.