Not at all superfluous
If you combine this strap with a face foam worthy of the name, you're finally on a VR headset that's pleasant to wear for longer gaming sessions. The battery extension is useful to solve the weak point of the Oculus (its autonomy) but with a big drawback in my opinion if you play connected to a PC via a cable: the battery extension can not be connected at the same time.
But it does the job!