Martin writes, "Insistence on written sources can be self-defeating in this context."

Nothing could be truer about the entire field of foklore, which in its "purest" form relies entirely on oral tradition rather than print.

As folklorists gradually discovered, however, the lore of industrial societies with a robust print culture (and later audio recording and transmission) increasingly flows from printed rather than oral tradition. (Not entirely, of course, but increasingly as time passes.)

Written sources can establish unimpeachable dates after which we know the cultural item existed. They give us solid reference points to make folklore study a "discipline" rather than just a hodgepodge of speculation.

Knowing "the earliest known date" also helps restrain baseless conjecture. If Item X appears in print in, say, 1885, it's entirely possible that it existed in 1884 or 1883. Rather less likely would be 1850, far less likely would be 1750. And any claim, without good evidence, that it existed in 1650, could be dismissed. (These dates are just examples in a simplified presentation. They're not magic or anything.)