Hi Gandalf,
It backs up everything that’s in the directory that contains wp-admin, wp-includes, etc. (except stuff that’s already in another backup, e.g. plugins, themes).
The main advantage is that you might have manually stored some other content in that directory, and need to make sure it is backed up. Or if you’ve modified part of WordPress core by hand, then you will want your changes backed up. Otherwise, it’s useful if you want to have a complete backup in one place. Other than that, it’s not hugely useful, since WordPress core itself can always be downloaded afresh from wordpress.org. See: https://updraftplus.com/shop/more-files
David