The key difference between islands and what we used to do back in the day (js on a static page) is that with an islands approach you architect your site with a components-driven approach where everything encapsulates the js/css/html it needs, then you mark it as an "interactive" island if you actually need client-side js to run – the code is the same, but it either runs only in the server (default) or in both server and client.

I know this sounds similar, but, compared to the more traditional approach, there is a certain simplicity to having everything just be javascript. You can often run the same libraries on both server and client depending on your needs, plus it fulfills the promise of web components in a way that is easier to work with (though WCs have also come a long way!)