Despite the modern use of Electronic Navigation Aids, the magnetic compass remains a primary navigation instrument on any vessel, and operates independently from any other means of navigation.
Over a period of time, or after certain events, the vessel's magnetic fields may change, altering the residual deviation of compass.

Solas requires that every vessel
has a well adjusted compass checked every year
(or every 2 years if there has been no significant change).

A few examples of situations when magnetic compasses adjustment is required.

"Magnetic compasses should be adjusted when :

a) they are first installed;

b) they become unreliable;

c) repairs or structural alterations have been made to the ship that could affect its permanent and/or induced magnetism

d) electrical or magnetic equipment close to the compass is added, removed or altered;

e) the recorded deviations are excessive or when the compass shows physical defects; or

f) at any other time deemed necessary by the master for the safety of navigation.

All magnetic compasses shall be swung and adjusted no less often than
- every two years;
- after dry docking; or
- after significant structural work."

Annex G of ISO 25862/2009(E)
"Adjustment of magnetic compass deviation"