When sitting in a plane we experience noise made by the motors of the plane.
After a short while we do hardly notice this noise anymore. This is due to the ability of our body to adapt to surrounding disturbances and thus cancelling out this noise.
The same holds true for magnetic fields, which pulse the whole time with the same frequency and intensity. These pulses are a disturbance for the body and it will adapt to the used pulsing frequency after just a few minutes and the effect will become close to nil (this the reason why some manufacturers call for the use of just 8 minutes) By automatically changing the used pulsing frequencies every couple of minutes this adaptation effect is avoided and effective therapy is thus obtained for the whole therapy session.