The only wiring diagrams I had were black & white scans of the factory service manual wiring diagrams, so I couldn't tell what color was what. I ordered the wiring diagram to have the wire colors in order to trace the car's wiring. (As an old car, there were wires added and rerouted).
The first thing I wanted to see was the wire to the radio... but the radio was missing from the diagram! I called in and reported the problem, and the diagram was revised quickly and an updated diagram sent to me in a couple of days at no charge.
So now I'm at the stage of trying to identify mystery wires that are on the car but not on the wiring diagram. If I find anything that appears to be factory and might be missing from the wiring diagram, I will call it in.