Complex sentence: One benefit was the high elevation angle of the DT machine gun in complement with high speed and good manoeuvrability, which allowed BA-64s to be used in urban combat with success against enemy infantry hiding on the upper floors of buildings.
Split into simpler sentences: One benefit was the high elevation angle of the DT machine gun. Together with the vehicle's high speed and good manoeuvrability, this made BA-64s successful in urban combat against enemy infantry on the upper floors of buildings.