You're supposed to use them to read fortunes.

[Akira shrugs, again pretending that his life has been less relentlessly intertwined with these cards than it has. And that he doesn't know off the top of his head what Chariot or Fool would look like in a person, both according to Chihaya's books and whatever power assigns Personas.]

I have a friend in Tokyo who sells tarot readings, so she talks about them pretty often.