When building a complex machine the last thing you want to worry about is incorrectly mapping out wires to the electronics that control and power the components. The second and third issues to worry about are intermittent connections and lastly cable-fatigue.
I worked on a project to ensure a reliable and readily reproducable electrical subsystem by means of standardized component, control board, and machine-routing wiring harnesses. The system allowed for foolproof connections throughout the machine. We were able to contract manufacture the harnesses because the design was sufficiently standardized. Additionally, since the harnesses allow for modular connections components can be easily added, replaced, or removed when needed.