The Mechanic slot on a ship or base is rather a unique one. Not only is the MECH in charge of the entire ship and/or station. They are also responsable for the upkeep, maintenance and repairs of the fighters and bombers. Not only for their own squadrons but for any rebel factions out there that may need repairs.
The MECH is also responsable for the dozens of junior mechanics that help maintain the equipment. They are in charge of all of the mechanical aspects of the numerous droids. Not to mention any other equipment in the base and/or ship.
Basically...if you can touch it and it operates...it is the MECH's responsability.