Albert Barnes Commentary Judges 9:44

"And Abimelech, and the companies that were with him, rushed forward, and stood in the entrance of the gate of the city: and the two companies rushed upon all that were in the field, and smote them." — Judges 9:44 (ASV)

This verse explains the purpose of both the present and the former division of Abimelech’s forces into several companies, namely, that while some of the companies attacked the men of Shechem in the field, another company, starting from their ambush, might occupy the approach to the city gate, and so cut off their retreat.