Here's a trivial example: https://supremecommander.ai. A raw CSS implementation of my blog's logo would have been a pain to build and maintain, but with Astro the code is relatively straightforward JS that becomes pure HTML/CSS at build-time.

The other nice thing is that you can throw all kinds of preexisting components from React/whatever into your site, and it will ship zero JS to the client until you explicitly flag a specific JS resource as an "island".

The only special thing about "islands" is that they're an escape hatch from the default behavior of JS being strictly build-time-evaluated. I found the terminology and description a little confusing at first too, because it makes it sound more special than it is. But the concept makes sense when you understand the context of Astro's intentional default behavior.