Okay, I may have answered one of my own questions as I’ve gone through more of the exercise.
The DEF-Base bone appears to be important once the mesh and Armature are connected. It creates a vertex group that the Crane Base can be assigned to, since the Root bone does not receive a vertex group. Otherwise, there’s nowhere for those verts to be assigned and their deformation would be incorrect when the rest of the rig is moving.
