Currently you build a static site like you do with Gatsby or Jekyll or any other static site generator. In the future it will support dynamic server rendering.
Doesn't almost every large front end framework have a static site generator? I think Vue for example has it built in for example.
Why would anyone use this over the already existing one in their current ecosystem?
Astro focuses on partial hydration where you only load the JS for the specific components that need to run in the client. Very few tools and none of the framework SSG's that I'm aware of do this.
There's also a variety of other features that Astro brings, the blog post talks about them.