I , for a long time have always wondered about this. It has never been made clear in any books I've read or documentaries I have seen.
The restaurants had to collect points in order to buy more stock for their kitchens. They took the points to the local ration board and exchanged them for vouchers that allowed them to buy quantities of food at a time.
Lots more here: http://library.duke.edu/digitalcollections/hfc/