The helmet protects the head. A person who is wounded in the head is out of the battle. Satan has deceived many with false ideas about God. Satan attacked Eve in the garden of Eden and tries to attack us in the same way. We are commanded in Ephesians ch6 v17 to take up the Sword of the Spirit and use it.
Prayer is more than just another weapon.
It underlines all the other pieces of armour. We must be a people of prayer, in direct communication with our commander. In all the armour there is nothing to protect your back. All the armour protects the front of us. There are probably at least two reasons for this.