Prime Type, Crown Type, Essence Type, and Conqueror Type - these and all existing Types, were created out of necessity, not for convenience in categorizing monsters.
Each Type was different in some way because if a monster was simply stronger or bigger than the others, threat level and white/red book were more than enough to determine the danger of the monster.
In a way, Type was a reward, a title that only the worthy could earn. Those who possessed something unique, towering above the rest.
Prime Type was the epitome of strength and power. Crown Type represented experience and countless battles, and Essence Type was pure firepower.
The Conqueror Type was somewhat unusual even among the other Types, as the main threat of this Type was not the monster but its subordinates.
Snow Imots were Blue Threat Level monsters, even a thousand of them were not capable of defeating dozens of Phantoms working together.