In the RAF of old, it was common practice to have a professional directional consultant as part of the crew - the navigator. He still survives on many, but not all, RAF flight decks.
It was normal, when using a slaved gyro compass in those days, for this gentleman to set the variation into the Master Indicator and update it as the aircraft crossed each isogonal. This meant that the SGC, although sensing magnetic heading, actually displayed True Heading.
He also usually had the latest wind already plotted on his CRP-5 (or military equivalent) or could get the drift off the Doppler. Therefore, since the compass read in True, on getting a QUJ, he could give an instant True Heading to fly for a homing to the pilot.
I don't think people use QUJs much these days.