Deuteronomy 1:38 (ESV)

38 Joshua the son of Nun, who stands before you, he shall enter. Encourage him, for he shall cause Israel to inherit it.

Israel would need a new leader to be appointed in Moses' place, to lead them into the Promised Land. Joshua, the servant/assistant of Moses, would be the one to take his place. God called Moses to encouraged Joshua for the task that was ahead of him, a task (causing Israel to inherit the land) that Moses often said how much he wished he could perform.