In addition, Extensis Fonts is always up-to-date with all of the available Google Fonts. Many users prefer it as it improves productivity and workflow - YMMV.
The Add-in is an optional menu that allows you to preview and try out different Google fonts directly in your doc without needing to return to the Google Font menu every time. You are absolutely right that Extensis Fonts displays fonts that are also available in the default Google Docs font menu – albeit with a couple of extra steps.
