When Joshua told Moses that he heard a sound of war, Moses was able to discern that the sound coming from the people was a sound of idolatry. Moses had been with God receiving instruction forward progress.
Intercessors have to stay on post and on guard because when you come up missing corruption will take over. The more you understand your prophetic or intersession gift and scope the more proficient you will be. When Moses God became tired he had to have intercessors on his left and right to help win the war.