Richard Dawkins interviews psychologist Randolph Nesse

My own Hypothesis:

It seems, after experimenting with my own blind spot a little bit, that it actual might be a photosenser that serves at least 2 distinct purposes. The first purpose has to do with collecting information about the intensity of the light that is hitting the retina, and thusly the brain must adjust the light sensitivity or contrast levels of the eye to register better in darker and lighter conditions.

The second purpose might have to do with dialating and constricting the pupils (perhaps also constricting and dialating the blind spot respectively), which is linked to attention and reaction by the sympathetic and parasympathetic nervous system. Mainly because, perhaps, the eye is a valuable asset to the whole body and can't afford to be poked out by a tree limb or flying insect.

quantum_flux is mistaken: the blindspot is so-called because it is a gap in the retina through which the optic nerve passes i.e. there are no photoreceptors in this region and it does not change size. The intensity of light reaching the retina is modulated by pupil size, the control of which is not affected by light incident on the blindspot.