God does not necessarilly need Mary to save the world. In fact, He does not necessarilly need the body of Jesus to save the world. Why didn’t God just decide to forgive Adam and Eve and pretend that nothing happened like he normally does towards other sinners? Because if He did, there would be no justice and He decided that only through Jesus can justice truly be delivered. But since Jesus represents God’sjustice, Mary represents His mercy and since Jesus came to this world thrugh Her, God wills it that we go to Him through Her.