The magnetic compass is used to navigate using either just the compass rose or a set of sights. As magnetic North is varying, the more professional compasses have an option to adjust for this variation by offsetting magnetic north from true north. The simpler compasses (mainly used in navigation) do not have this option but rely on the user correcting the instrument themselves.
Apart from the variation steel or iron objects will also attract the compass needle. The instruments have no adjustment for this as this value depends on the location ánd the direction the instrument is pointing in as the steel object itself becomes magnetic and acts as a 'north pole'.