The magnetic head, or slider as it is commonly referred, does not actually touch the disk's surface while the disk is spinning. Instead, as the disk spins air is forced under the slider forming a very thin cushion allowing it to rise and fly much like an airplane. The suspension's role is to apply force in the opposite direction in order to maintain the ideal distance between the head and the disk for reading and writing data, and the reliability of the data. Additionally, the suspension maintains the flying attitude of the slider by controlling and restricting its range of movement.
Some interesting facts about the flying height
Modern suspensions allow the head to "fly" above the disk surface at a height of less than 10 nanometers and the readers and writers are typically at 1 nanometer during the reading and writing of the data. Using a comparable flying height to mass ratio this is equivalent to a Boeing 737 airplane flying 0.5mm off the ground!
A modern disk drive's flying height is smaller than the circuit size of today's most modern microprocessors.
The slider is moving at speeds relative to the disk ranging from 30 to 160 kph.