Why were gyrocompasses especially useful on ore ships on the Great Lakes?

Gyrocompasses are especially useful on ore ships on the Great Lakes because they are immune to magnetic interference.1

Unlike a magnetic compass, which relies on the Earth's magnetic field and can be deflected by local magnetic influences—a phenomenon known as deviation—the gyrocompass uses a continuously driven gyroscope to seek true geographic north. Because ore ships carry large quantities of iron ore, the presence of these deposits, along with the ship's own steel structures and electric circuits, would cause significant errors in a magnetic compass. By utilizing a gyrocompass, navigators on these vessels can maintain an accurate heading regardless of the magnetic disturbances caused by their cargo or the ship's construction.1234