The thing that's best about GitBook is the way you can organize a structured outline of the site's contents. The other killer feature is it's ability to do code-related formatting, including:
an editor almost as nice as Medium (and more featured)
contents/outline with nesting and collapsing
code blocks with language-specific highlighting
tabs (e.g. to provide examples in different languages, platforms, etc.)
This is an example in case one.
This is an example for another case.
API documentation form
variants (e.g. different spoken language versions of a page)
integrations: Slack, GitHub, Google Analytics, Intercom
versioning of edits (as in why it's called 'git'book)
the free gitbook.com service also allows using a custom domain name