Originally Posted by Ko.Fe.
This is how most battery grips work.
Well, the other idea is the way it's done on the Nikon FA, which has a set of contacts in the bottom plate that mate with contacts of the motor drive.
That way, when the MD-15 drive is fitted, you don't need to install the camera's own battery. But since the M9 has no such contacts on the bottom, well . . . you get the idea.