Then Jerubbaal, who is Gideon, and all the people who were with him, rose up early, and encamped beside the spring of Harod: and the camp of Midian was on the north side of them, by the hill of Moreh, in the valley.
The text does not enable us to understand God’s motive certainly. Simple obedience is what He required. Before God told Gideon to let the larger group of soldiers go home, He gave him a promise that He would deliver Israel with the 300 remaining warriors. This promise undoubtedly encouraged Gideon’s faith.