Charles Ellicott Commentary Numbers 31:27

"and divide the prey into two parts: between the men skilled in war, that went out to battle, and all the congregation." — Numbers 31:27 (ASV)

And divide the prey into two parts ... — It was reasonable that those who had encountered the perils and hardships of the war should receive a larger share of the spoil than those who had remained in the camp. It was equally reasonable that the latter should not be left without some substantial benefit from the victory miraculously achieved over the enemies of the Lord and of His people. A similar direction is found in Joshua 22:8 in regard to the division of spoil taken from enemies, but no general ordinance appears to have been made in regard to the division of the spoils of war until the time of David. (See 1 Samuel 30:24–25.)