Those coins used to be used to purchase tokens that would augment the tome gear of that tier. But I've discovered that you can buy the level 60 gear already augmented now (not sure on the lvl 70 stuff) and I think you can trade them for other trinkets as well (music rolls, etc). Check Idyllshire for coins from the void raids and Rhalgar's Reach for the coin set from the Ivalice raids. See what they have to offer.