The magnets are used in the mechanism that positions the heads. The magnets provide a strong magnetic field.  Within the magnetic field is a coil. In order to position the heads, a current is passed through the coil generating a magnetic field within the coil.  The interaction of this magnetic field with the permanent magnetic field  generates the force that moves the heads. By varying the current in the coil, the heads can be precisely placed on the disc. In order to move the head mechanism very quickly from track to track, a very powerful magnetic field is required. If the magnetic field is weaker, there's less force moving the heads and the drive access time will be slower. Since drive manufacturers strive for the fastest access times, they must use the most powerful magnets they can find. Drive manufacturers have made great strides in making the read write heads and mechanisms much smaller and lighter. This means the later drives need much smaller magnets but they still need to be very strong or access times will suffer. The scientific reader will probably wonder why electromagnets aren't used instead of permanent magnets for the magnetic field. The answer is probably that heat production and power consumption would be dramatically higher with electromagnet use. Still, it was a good thought and I'm glad you thought of it.

These magnets are a little different from what would be expected.  They're actually polarized with the poles on the flat surfaces of the magnets rather than at the ends.  Most magnets will be "N" at one end and "S" at the other.  These will have "N" on one flat surface and "S" on the other.