Yes, they do. They supply the base functions supplied by factories - AAA Factory (auto guns/ack), Ammo Factory (ordnance), Training Facility (barracks/troops), Fuel Refinery and Radar Factory.
One load reduces rebuild time by 30 minutes. Destroying half the convoy or barges reducesthat load to just 15 minutes reduction of rebuild time.
They don't affect hangers, manned guns or town buildings.