The last, and probably one of the most often used tools in your SharePoint Framework toolchain, is the SharePoint Workbench. This HTML page is served from a local filesystem by Node.js and loaded in a browser instance when you run gulp serve.
By default, SharePoint Workbench is dynamically loaded as workbench.html locally and is only accessible from your localhost. In addition, SharePoint Workbench also exists in SharePoint and can be used against a real Office 365 tenant or SharePoint 2016 to see how your code truly works.
In a typical development workflow with the SharePoint Framework, you'll mostly use the locally hosted workbench.html to test and see how your code works. When you're ready to package and test your code against SharePoint, you'll be using the hosted version from SharePoint 2016 or Office 365.
It's worth pointing out that the local version of SharePoint Workbench (workbench.html) does not require SharePoint installed locally....
