To keep valuable

Iron is a ferromagnetic material and therefore has high value of magnetic permeability (μ) A ferromagnetic substance is strongly attracted by a magnet therefore all the lines of earth magnetic field pass through the surface of iron box and no flux pass through the valuable instrument.

Thus, the magnetic field inside the box becomes zero and there is no harm to the instrument.

Therefore, due to ferromagnetic nature of iron it is used to protect valuable instruments.