Lots of things are possible with access to source code that are still easier when someone writes a tool for that scenario.

Crawling a build you already have isn't one of them

The website in question may be a dynamic website (e.g., WordPress, MediaWiki, or whatever other CMS or custom web app) and you either want a snapshot of it for backup, or you run it locally and want un static copy to host it elsewhere that only support static files.