The HDD must be able to maintain the read and write elements of the head over the data with a position error of less than 10% of the track spacing. Positions errors beyond this limit while writing can begin to cause errors on adjacent tracks by overwriting (destroying) portions of the data that was written there. Similarly, when reading, if the head goes off track the head will not read the correct data and the data will have to be reread slowing down the drive and the flow of information to the user.
With the latest generations of suspensions a small PZT actuator is built into the suspension giving the drive a dual stage actuator. Seeks (moving the head from track to track) and large scale track following are still controlled with the VCM/pivot/e-block actuator while the PZT actuator on the suspension does the fine control.
To give a feel for how closely the head is controlled in the drive a comparison with an automobile traveling on a highway is a good comparison. The head is moving at automobile-like speeds (30-160 kph). If the head was the same size as a car it would be able to stay in a traffic lane that was 0.04mm wider than the automobile or approximately the thickness of the clearcoat that is over the paint wider than the automobile on each side.