Trade cards

Trade cards were handed out as advertising souvenirs at all of the major stores and products. Trade cards were also handed out by merchants or left on sales counters free for taking.
Many sets of cards are, in effect, mini-encyclopaedias covering a multitude of themes. Some are interesting from an informational standpoint whilst others are simple attractive to look at. To someone with a strong collecting streak the sets offer neatly packaged (and, quite importantly, achievable) targets to obtain which themselves can be developed into one or more thematic collections. Moreover, Cartophily is a well-established and reasonably well documented hobby.