The essences were a rare resource that the Phantoms hardly ever used. Mostly, the essences were necessary for young Phantoms to improve their artifacts.
Oddly enough, it was the scientists and the government that utilized the essences the most. It was a capacious source of energy, rather unstable, but the government had its methods of dealing with this drawback.
Artifacts were more stable, but it was harder to get energy out of them than the essences. So, when the Phantoms took a mission to hunt the Essence Type monsters, they were harvesting essences for the government.
A lot of neighborhoods and factories were powered by essences. It was a valuable resource that was useless to both the Phantoms, who chose their own artifact, and the common people even if they had enough money to buy essence.
Sure, monster parts, artifacts, and even fresh shards of the Midnight Crystals were used throughout the Citadel. Each had its use, but essences only had one: energy.
