A repairman is a unit from the original game, The Conquerors. They are used often in defensive strategies.
Using a repairman in the early game could be good for games with no truces available. 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! Also they can heal all naval units.
While in a battle, they are useful since they can heal aircraft carriers destroyers and submarines, even if you try using a transport plane to get these guys on oil rigs. They can repair them too and they are useful when a base has been nuked or raided since it would be hard for players to attack if you are doing a defensive strategy. If you do not need any heavy soldiers or juggernauts, and do not plan to, use 14 of these and play defense and spam repair.