I seem to recall sometime during the 1980s that there was some kind of a REQUIREMENT that card packs include 'something of value' (gum, stickers, puzzle pieces, what have you). For example, Donruss' 1982 product was "puzzle and cards"...
...and I've always wondered why cards couldn't be sold on their own merits, as they are today. (And if this was the case, how was Topps able to get away with selling rack packs which had nothing BUT cards?)