Adam Clarke on Judges 20:32

Let us - draw them from the city - They had two reasons for this:

1. They had placed an ambuscade behind Gibeah, which was to enter and burn the city as soon as the Benjamites had left it.

2. It would seem that the slingers, by being within the city and its fortifications, had great advantage against the Israelites by their slings, whom they could not annoy with their swords, unless they got them to the plain country.