What's the difference with, any webbrowser on a computer, File -> Save as ?

Save As works fine for simple websites with static content.

Let's say you have a site that fetches content from a database. If you Save As, then at best you'll get a local copy of an HTML page with JS that loads the content from the same remote database. It might not work (since the local copy has a different origin), or if it does, it requires you to be online, which defeats half of the purpose.

What this project, and SingleFile, both do is save a snapshot of what the rendered page actually looks like at that moment in time. The scripts are stripped out so it runs locally and has no external dependencies.

That's for a single page, this handles the whole site. Also the browser Save As options often work poorly.