Moses was not only the leader and ruler of the people, he was also the medium through which God communicated with the people, gave them his laws, and conveyed to them his word and will. In this respect his place could be supplied neither by priest nor by king.
In the prospect of his demise, therefore, there required to be instituted another office, that of a prophet, one who should be between God and the people, as the channel through which Divine communications might pass to them.
This office Moses here announces that God would establish among them when they had entered the Promised Land.