Airplane spotter playing cards were a deck of cards issued during World War II to soldiers, so while they pissed away their free time gambling away their paychecks, they might learn a thing or two about what friendly and enemy aircraft looked like, and hopefully could therefore tell whether that huge chunk of speeding iron flying through the air was on your side or not before you could see the markings on the side of the fuselage. There surely must also be some men who studied the cards prodigously, intent on learning to identify every plane as soon as it was seen, and they may in fact have been quite numerous.

But as far as good logic goes, putting the silhouettes on a deck of cards was a stroke of genius. Not only is it something that soldiers have been known to frequently use in their free time anyway, but it's an awful lot easier to carry a deck of cards than a giant chart or poster, and easier to flip through than a little book, and has no binding to ruin.