What are Dragonbone, Dragonglass and Dragonfire Keys for?

The 3 keys open the dragon chests at the Hatchery:

• Dragonbone — the most common.
• Dragonglass — the middle one.
• Dragonfire — the rarest, and the best chests.

Chests are the main source of two crucial things: the dragons and relics themselves, and above all the habit materials — the Breedmarks (per breed) and the Cores (🔵 Rare / 🟣 Epic / 🟡 Legendary).

Since all of that is permanent and never resets between reigns, farming keys and opening chests is definitive progress for your dragons. It's the best long-term investment in the game.