What is a reservoir bag used for on a standard non-rebreather oxygen mask?

Possibly one reason could be to help monitor respiration. This could provide a visual clue to inspiration. But, the reservoir bag is to collect oxygen and thus keep a free flowing "reservoir" of oxygen for the patient to draw from as they inhale all the avilable air within the mask itself. There are exhaust valves located on either side of the face mask to exhale the CO2 and keep from drawing in room-air. Or even rebreathing the CO2.