Because the ONLY heading refrence you have in the cockpit (the compass) is magnetic. Because the dg is subject to gyro drift/precession it needs ro be reset or synched regularly.
The only refrence you have in the cockpit to refrence and synch with is magnetic. Hence magnetic is always used.
Modern nav systems (inertial/gps etc)
can use any refrence however the normal convention is still to use magnetic.
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