Verse five shows us that he has authority over the general populous and verse six shows that he also has authority over the priests. So, positionally Joshua was already a leader doing his job. However, this passage marks a new phase in his leadership. He already had the respect due his position, respect born of years of faithful service under Moses and the historic victory as one of the spies that went into the land. Either of these other avenues would have resulted in largely the same end result and yet the communication came directly to Joshua, just as it would have done to Moses.
Leadership has both internal and external qualities.
Externally, we hold office and perform our duties. People often respect the position and what that represents and not the person who holds the position. Similarly, the person holding a position of leadership will often trust in the external trappings of leadership - uniform, badge, job title, privileges, etc. However, relying on purely external trappings of authority leaves us empty. We inhabit a shell that is too large for us.