To clarify with these pieces - the photo or picture card was stuck on the reverse of the notgeld itself. It created yet another set of the notes for a collector to try and obtain. The oval-stampmark on the picture card side gave them authenticity and validity. The number of sets created like this could also be controlled, again making them a 'great' addition for a collection. Collectors always like to have 'limited sets' or sets where the actual print run / creation is going to be low. Demand and supply.....and scarcity.