There is also a tithing structure set up from top to bottom that's basically the same as a monarchy. The Hive have to feed the worms inside them to keep from losing their minds. Basically the thrall feed themselves and tithe some of their power to the acolytes. The acolytes do the same for knights and wizards. They tithe up to the yellow-bars. They go up the chain as well, and eventually end up at Oryx/Crota/whoever is their "god". There is also a promotion strategy tied to it. If you're a thrall that can kill enough to tithe and feed yourself a ton extra, you can evolve into an acolyte. Same with acolyte to knight/wizard and so forth.

Purpose. The Hidden Swarm is the general Hive brood on the Moon, the Spawn of Crota are descendants of Crota, the Blood of Oryx are descendants of Oryx, and I have no clue what the Hidden Sect do. Perhaps they are the wardens or guards of the Hive