Rapier is a heavy sabot rocket gunship. It has very good armor, able to withstand most of T1 anti-air defenses. Their most common use is for defending your base or protecting your teammates within your air territory. They are not effective as striking units, due to their low speed. For this purpose - use bombers, or Blades. It is less armoured but has a better weapon than its ARM counterpart (Brawler).