Albert Barnes Commentary Judges 20:10

"and we will take ten men of a hundred throughout all the tribes of Israel, and a hundred of a thousand, and a thousand out of ten thousand, to fetch victuals for the people, that they may do, when they come to Gibeah of Benjamin, according to all the folly that they have wrought in Israel." — Judges 20:10 (ASV)

In order to make it possible for the Israelite army to remain in the field, and do to the men of Gibeah what their wickedness deserved, every tenth man (40,000 in all) was appointed to find provisions for the whole army.