Ross G Caldwell said:
But to ask the "why", is to ask the unknowable, at this point.
Ross
As someone who makes cards, I'd guess (but it's a guess) that the answer lies in the size of the paper sheets used. Probably there was space for an extra card so something was invented to fill it. It's still an issue in tarot card production today - 78 cards leave two spaces on the sheets. So one is a title card and the other?...
Of course, they were not doing lithography, so the issue of space on the paper would work quite differently. But when you see a "wild" card introduced I'd tend to look for very practical production reasons rather than anything else. Why waste expensive paper when you can add something of value?