In other words...
In other words, as soon as Moses (as) was commissioned to the Divine Messengership, he demanded four things from Allah: The expansion of breast: “My Lord!
Expand me my breast!” A preparation for the conditions: “And make easy for me my task,” A fluent speech: “And loose a knot from my tongue,” Having an assistant: “And appoint for me an assistant…” And since Moses (as) had been commissioned to different subjects, such as: worshipping, political, revolutionary, social, ethical, personal, and congregational affairs, and paying to every one of them naturally causes man to neglect some others, therefore, the expansion of breast is utterly necessary for obtaining them all.
“And associate him (with me) in my affair.” Since carrying on the heavy responsibility of prophethood toward the final point needs someone to aid so that it is impossible to afford it alone, then the fourth request of Moses (as) from Allah was that he said: “And appoint for me an assistant from my family,” Then he mentioned his brother in particular, and said: “Aaron, my brother;” Aaron was the elder brother of Moses.
He was three years older than Moses, with a tall stature and a well-proportioned body, and with a very eloquent tongue and an excellent apprehension. He passed away three years before the death of Moses. Aaron was one of the appointed messengers whom Allah bestowed on Moses (as) in a way of His Mercy. Then Moses stated his purpose why he asked Allah to appoint Aaron as his minister and assistant.
He said: “Strengthen my back by him,” And, in order to complete this meaning, he continued saying: “And associate him (with me) in my affair.” Moses asked Allah that Aaron associated him both in the rank of Messengership and in accomplishing that great mission. Thus, Moses did not ask Allah, only for the ministry of Aaron, but he also requested for his prophethood. Yet, in all respects, Aaron was a follower of Moses in all affairs, and Moses was his leader.
The reason why it is said in the verse ‘minister’ is that a minister carries out the heavy affairs and duties of the guardian of the affairs.