I thought they were to be used when storing the belt for any reason as the belts mark the pale plastic on top of the seats when they are not in use if they are left rubbing.
The top of my rear seats were a horrible grey/mucky colour from the belts resting on them. It also cuts out an extra source of noise from the rear seats.
I have not seen anything to suggest that they should only be used when the seats are folded or unfolded... Why should they not be used for storing the belts anytime you are not expecting to use them?