There is no documented use of their significance prior to when they started being used in the 18th century as a divination tool. There have been writings that supposedly traced their symbolism to ancient Egypt but this has never been verified.

Edit- A lot of people assume they represent something as the European suits do, but the Europeans simply added them for either vanity or gaming purposes.

A good source for any discussion of the Tarot is Arthur Edward Waite (co-author of the Rider-Waite deck), especially see The Pictorial Guide to the Tarot (section Four, titled "The Tarot in History," will be especially useful). Waite claims, I think persuasively, that there is no evidence for the Egyptian origin myth. Rather, he states that the appearance of Tarot decks (or similar card collections) in a multitude of ancient cultures simply illustrates "the universal tendency of man to be pursuing the same things in more or less the same way" (ch.4). Here and elsewhere, he writes that the Tarot appeared first in Europe in the 14th century, as a way to allegorize the great spiritual truths of what Waite and his fellow hermeticists simply believed was "The Tradition" (a quasi-gnostic blend of Western esotericisim and Christian mysticism). So short answer would be: according to this reading, the cards were never just "cards"--rather, the opposite is true. They have become simply playing cards, but this is not the purpose to which they were first designed.