This is because the eye is moving side to side just enough to be able to see what would be in the blind spot. These rapid movements also prevent habituation of the photoreceptors. If the eyes did not move rapidly the neuronal signal from the receptors would fade then stop, and we would be effectively blind until the eyes moved again. Smell, taste and touch habituate, but the consequences are not so severe. Kinesthetic receptors take an extremely long time to habituate, so we can continue to move or maintain a posture.
Sight often involves stationary stimuli, so the system itself moves.
Another very interesting aspect of saccadic eye movements is that they unconsciously guide our eyes over the most informative contours of an object even before we consciously know what it is. This is also mediated in part by the frontal eye fields. One aspect of this knowing where to look before you consciously perceive occurs in crowds, when you turn your head and look directly at someone who is looking at you, without really knowing why you turned to begin with. Since the optic tract and the secondary and tertiary regions of the visual system are so extensive. Focusing from one object to another is often difficult or slower, as is visually scanning an object, reading or looking for something.