The Start Workbench is not really a workbench, it's just the page that is presented when you open FreeCAD with no document loaded.
From this interface you can see useful information and jump to recently opened files, examples, the documentation, and latest news from the FreeCAD world. You can change how the Start Workbench behaves, or change the default workbench, in the Preferences Editor in the menu Edit → Preferences → Start.
Start workbench, documents tab
Start workbench, help tab
Start workbench, activity tab