The repairman is a unit from the original game, The Conquerors. They are used often in defensive strategies.
Using repairman in the early game is not recommended, due to the fact that they take up room which can be used for soldiers. Later on while the game is halfway progressed, they are good for repairing damage from attacking units and they should mostly be around a main base at all times! Repairmen can heal naval units, while hospitals and medics can't.
While in a battle, they are useful since they can heal aircraft carriers, destroyers, and submarines. Repairmen are useful for recovering against nukes and large attacks. If you do not need any heavy soldiers or juggernauts, and do not plan to use 14 of these and play defense and spam repairmen.