16 And he shall be thy spokesman unto the people: and he shall be, even he shall be to thee instead of a mouth, and thou shalt be to him instead of God.
This verse gives one of the clearest examples of divine investiture and delegation. I’ll work through it backwards. Moses was told that he is to be to Aaron in the place of God. This wasn’t to say that Moses was God. This wasn’t even to say that he would be infallible, we know that he still made mistakes, such as when he struck the rock at Horeb. But from now on, God was going to speak to Aaron through Moses, so if Aaron wanted to have a full relationship with God, he was going to have to heed Moses.
The role that Moses was being called to fill was both old and new. Long before Moses there had been the patriarchs who stood as representatives of God to their sons. Now, though, the Israelites were a multitude of separate family units. They could not literally all have the same father, but they still needed a divine father figure. Moses is the first person we hear of that was chosen to fill this position. He was called to be a presiding prophet. Yes, every Israelite was still going to be commanded to seek the Lord on a personal basis, but that relationship was going to be cultivated, in large part, by following the directions of Moses.
And this is perfectly appropriate. Obviously, God is entitled to set whatever terms He chooses for how we will receive His light, and for the Israelites He had decided that a significant portion of that light would be channeled through Moses.
But just as God was going to be represented in the figure of Moses, parts of Moses would be represented in others. God would direct Moses in what to do and say, and Moses would delegate that “saying” part to Aaron. This idea would be further expanded on when Moses, at the suggestion of Jethro, delegated the judging of the multitude to a hierarchy of worthy men.
So, taking these two principles as a whole, the presiding prophet was specially selected to represent God by directing the affairs of the covenant people. That prophet was able to spread that responsibility out to others, and thus an entire body of people could become responsible for executing the will of God on the Earth. This was a new system of society and government, a gift of structure and procedure from God. Presumably we had not heard of this system previously among the Abrahamic nation because they had not yet been large enough to require it. Now that the system had been introduced, though, it would persist throughout much of the Biblical record.