When you shoot a fallens head off its ether/their soul escaping. Captains have the most ether and the servitors supply the ether. Thats why you see them gathered around Sepiks Prime. When you shoot a cabal its the fuel for their jetpack stuff that comes out, which is why when you go on their ships its on the floor and what not. They are not evil like the vex or hive imo. Fallen are just scavengers and the Cabal are just a military going through the galaxy. I believe the Hive are the real army of the darkness and the Vex just want to destroy everything or they were corrupted by the darkness and started worshipping it like they were doing with that ball of darkness ( the heart ) in the black garden. But good theory man