Any time you get or spend Gold, Gems, Champion or Legend Cards; some of them are stored in the Treasure Hoard. Making an in-game purchase allows you to open the Treasure Hoard and claim the items inside.
If you allow the Treasure Hoard to fill up before opening it, you will earn extra rewards. The Treasure Hoard gradually fills up as you earn rewards while playing. Once you decide to open it, you can decide whether to purchase the regular Treasure Hoard or the Dragon’s Hoard. The Dragon’s Hoard will grant you five times the rewards of the regular Treasure Hoard, and earn you an extra Mythic Chest.
After a purchase, the Treasure Hoard will empty and be reset.