The pole pieces are used in a pickup for directing the magnetic field to the strings. If the strings are off set from the pickups the signal or output can be affected. The relationship of the strings to the pickup is important as well as the shape of the coil. The magnetized string movement will move the magnetic field back and forth through the coil and the relationship is important for a balanced output form string to string. The pickup can still work with an off set string but may not be as efficient as a string directly perpendicular to the pickup.