Astro is a metaframework that among others can use Svelte for both static and interactive components. Plain Svelte without a metaframework is not suitable for normal websites with multiple pages, but Astro is.

As for Astro vs SvelteKit, I generally prefer Astro's approach to filesystem based routing (no +page etc.), I prefer it's MPA approach to the clientside routing SvelteKit and others use OOTB, I like that it's very tailored towards content and being an SSG (content collections, MD and MDX support ootb, currently experimental automatic image optimization), and the "integrations" are a huge time-saver. Some common ones you can add with `astro add`, and many others are just a quick config edit away. I recently built a pretty performant portfolio site with Astro in a single day using DecapCMS and UnoCSS, both as integrations, all the client had to do was accept the invite from Netlify Identity and start adding content to their site!

No advantage if you just want to use Svelte when SvelteKit does everything Astro does and more while having less dependencies.

Well, dependency count is not the metric on which I base my tech decisions on. After all, you could do everything Svelte does with VanillaJS.

I just prefer the DX of Astro and MPA routing instead of client-side.