The Eldritch Abomination (henceafter referred to as "Father" for convenience) needs a human vessel to effectively host his power.
On his own, "Father's" power is too otherworldly and erratic to be effectively used in our dimension. He can only act efficiently when possessing a human host, which focuses his power. The three "angels" act as channelers for "Father's" influence. Destroying them distrupts the possession, which makes Father's power unstable. If all are destroyed, he becomes unable to sustain his physical avatars and collapses.This theory explains why, despite reality-warping power, "Father" doesn't destroy the protagonist. The monsters are not specifically sent after her, they are simply agressive and wander around attacking anything non-demonic that moves. "Father's" goal all along is to lure the protagonist into killing Harmonia and make her the new vessel. This may be because Harmonia's body is worn down from hosting "Father's" power, as well as mothering his demonic offspring.
