Century: Age of Ashes is a free-to-play multiplayer game in which players fight from the backs of dragons. As you play, you will progress and level up in-game. There is a way to increase the rate at which you level up—this is with XP Boosters.
There are two ways to get XP Boosters. You can either earn them at certain levels by leveling up or you can purchase them through the store. These are purchased with Gems, and these are the boosters offered in store.
Leveling up is the easiest way to unlock more gear. This includes cosmetics, coins, gems, dragon eggs, and XP Boosters. Using XP Boosters will speed up the unlocking process and let you get more gear. So if you are looking for something to spend your currency on, then definitely consider XP Boosters!
For more information on Century: Age of Ashes, check out All achievements in Century: Age of Ashes and How to lock on and shoot an enemy with a fireball in Century: Age of Ashes on Pro Game Guides.