Even though technically Jesus doesn't have to have a purpose to exist, it sure was convenient that Yahweh was/had a sacrificial "Son" handy. Imagine if he'd been a Duality with just Father and Holy Spirit. "Well, I can't go, because I'm the one whose wrath the sacrifice is supposed to appease, and I can't send Him, 'cause crucifying the Holy Ghost would be like trying to nail Jell-O to a wall. Whelp. Guess the Devil wins."