In 1980, DC wanted to make some fuzz about the extra content pages they were adding to their books. The back-up stories were supposed to feature a whole stable of regular also-rans (if you don’t count Batman and Robin), and for some reason, it seemed like a good idea to sell all this as a stamp collection. You’ll want to own all backup stories, just like you want to have your collection complete, get it ?
I don’t know whether the inspiration for this were the Marvel Value Stamps of a few years earlier, but in any case, it was only an ad campaign. With cheesy copy to boot.