Read it again. Get it? Read it AGAIN. The half inch plywood gives a consistent nailing base. You can put a roofing nail ANYWHERE and have it hit the minimum required half inch material to hold the nail. Without the plywood, you have a roughly 25% chance of not hitting wood at all, and a high chance of just catching the edge, splitting the wood and not holding the nail..
The lath gives the roof strength. The plywood seals it and holds the nail.