They are using them more as medium or close range hybrid infantry support/artillery with 100mm High Explosive shells. Unlike traditional artillery, which is a very soft target, these T-55s can maneuver into position with infantry and use their large guns to take out hard targets and lighter enemy vehicles. Plus, each T-55 has a heavy machine gun mounted for both air and ground defense. Overall it's an interesting strategy to use otherwise outdated vehicles, but I'm not sure how effective it will be.