The Rocket Launcher (“RL”) is a simple, cost-effective defensive unit. Operating on the same principle as common ballistic firearms and requiring no further research, its low manufacturing costs make it effective against small fleets. In the early game they are used to deter overnight raids by weaker players. In massive numbers, they can be a deterrent even to larger fleets, making an attack unprofitable. They are also used as a fodder unit to protect bigger, more expensive defenses.
Note: For pros and cons of defensive structures, see Defense Comparison