Due to the earth magnetic field, your compass always seeks the north pole however, it can also be seriously affected by the magnetic field created by your own ship or electrical devices placed close to it causing a compass deviation.
Swinging your compass while the ship is underway and running courses on different headings, we cop calculating the compass deviation on each course so we can eliminate or minimize it, fixing correctors if needed. The remaining deviation readings are recorded on a Deviation Card.
Most national maritime authorities as MCA or RINA and the International Maritime Organisation state that Magnetic compasses should be adjusted and Deviation Cards updated every two years (ISO 25862) When the compass is initially installed, after dry docking and after significant structural work.
In some Flag states it is compulsory to update the Deviation card more frequently.
Let me remind you that magnetic compass is the only navigation equipment that can be used when electronic navigational aids fail and all other means of navigation are not available.
That is why national maritime agencies specify that vessels have their compass adjusted only by a qualified and authorised compass adjuster, therefore MCA state that vessels should be fitted with a properly adjusted standard magnetic compass (LY3 18.1.1)