What was the value addition of Moses' speech deficiency to his leadership?

We read in Ex 4:10-15 (KJV): > And Moses said unto the Lord, O my Lord, I am not eloquent, neither heretofore, nor since thou hast spoken unto thy servant: but I am slow of speech, and of a slow tongue. And the Lord said unto him, Who hath made man's mouth? or who maketh the dumb, or deaf, or the seeing, or the blind? have not I the Lord? Now therefore go, and I will be with thy mouth, and teach thee what thou shalt say.And he said, O my Lord, send, I pray thee, by the hand of him whom thou wilt send. And the anger of the Lord was kindled against Moses, and he said, Is not Aaron the Levite thy brother? I know that he can speak well. And also, behold, he cometh forth to meet thee: and when he seeth thee, he will be glad in his heart. And thou shalt speak unto him, and put words in his mouth: and I will be with thy mouth, and with his mouth, and will teach you what ye shall do. Later we see Aaron always accompanying Moses except on a few occasions like the receipt of the Commandments. God could have easily removed Moses' deficiency on Day One, but ordained that it stays. The fact that Moses was instrumental in the successful redemption of Israel from Egypt proves that his speech deficiency did help him. My question is: What was the value addition made by Moses' speech deficiency to his leadership?