When and why do designers use pole pieces?
Magnetic flux will travel along the path that offers it the least amount of resistance, (or, more accurately, the least amount of reluctance). Steel components in a magnetic circuit offer the flux a low reluctance path. This fact allows the use of steel pole pieces to capture flux and concentrate it, (or merely redirect it), to the point of interest.
Focusing of flux can be achieved by tapering the steel. However, one must be aware that as the pole area of the steel pole piece decreases, the flux density within the steel (B) will increase (if the total flux traveling through the steel component remains constant). At some value of B, (typically around 18 to 20 kG), saturation of steel may occur. When saturation of steel occurs, the flux passes through the steel as if it was made of air, leading to losses in the magnetic circuit. Steel pole pieces can also be used to homogenize the field over the active volume in, say, a dipole.