Joshua had been Moses’ assistant for more than 40 years. With Caleb, he’d seen the promised land and believed God could give it to the Israelites, but hadn’t been able to sway the rest of the community to his point of view.
Now that generation had gone: Joshua was to take Moses’ place and lead the Israelites into their new country:
Numbers 27:18-21Take Joshua son of Nun, a man in whom is the spirit of leadership, and lay your hand on him. Make him stand before Eleazar the priest and the entire assembly and commission him in their presence. Give him some of your authority so that the whole Israelite community will obey him. He is to stand before Eleazar the priest, who will obtain decisions for him by enquiring of the Urim before the Lord. At his command he and the entire community of the Israelites will go out, and at his command they will come in.’
With this, the new way of approaching God starts: no longer would there be a Moses to talk to god directly. Now the political leader needed to approach God through the priests, starting with Aaron’s son Eleazar.
What would have happened had Moses not been such a reluctant speaker more than 40 years earlier when God commissioned him at the burning bush? Aaron wouldn’t have been needed, and this whole high priest thing—with a separation between political and religious leadership—could have been arranged differently.