As an Index controler and Pico user: The back straps are pretty much essential for any serious use; the controller purportedly includes finger tracking (capacitive) but you can't really open your hand without dropping the controller unless you have the strap.

If as I currently intend I end up purchasing this device, I will definitely endeavour to obtain the controller straps as well as the top strap for the headset at the same time, and I recommend others do the same.

It also includes the spacer for wearing glasses.