It may not be in the documentation, which you could raise with Netlify.
I determined it earlier by doing a quick test, creating one site instance with drag & drop and another connected to a git repo.
The only obvious “Note” that I see doesn’t mention linking to a git repo as a requirement, but it does lead to git related documentation:
Builds must be active for build hooks to trigger builds of your site.
If your site was in a repo and linked, then you would be able to either trigger builds via the build hook (e.g. when a content author uses the cms), or when you push changes to the code.
Usually this is precisely what you would want, the developer keeping the code up-to-date as needed and the content author rebuilding when they change content.
In regards to draft content etc, it’s up to you and your specific project circumstances which content you include in your build.