The Workbench engine in 2.8, is Blender’s solid/wireframe viewport shading mode. That is to say, even when you’ve chosen the Eevee or Cycles engines in your Render properties tab, whenever you’re using solid shading mode in the viewport, you’re using the Workbench engine. I suppose the devs made it accessible in the engine menu for completion’s sake, but there is no reason to ever select it there – and as you found out Omar, if you do select it there, the other shading modes become unavailable.
ETA: Oh wait! I -can- think of a reason to select the Workbench in the Render properties menu! You would want it there if you want to render the solid-view viewport, either as an image or an animation. This would be the equivalent of using “OpenGL Render Image” or “OpenGL Render Animation” in 2.7x, except that it will still use your camera.
