About.com suggests that blind spots have an explanation rather than a function. The explanation is that the part of the eye that the optic nerve exits from does not have rods or cones, thus resulting in a blind spot. Scholarpedia states that blind spots are present only in vertebrates because eyes in vertebrates develop from the brains, as compared to cephalopods, whose eyes develop from the skin and therefore have no blind spots.
Scholarpedia goes on to say that there are a couple of hypotheses on why people do not notice their blind spots, also known as scotomas. The first hypothesis is that the brain fills in these blind regions, which suggests that both memory and imagination affect vision. This filling-in theory postulates that the brain adds objects, patterns and colors to the blind spot. The second hypothesis is that the brain merely ignores the blind spots.
However, Reference.com points out that blind spots beyond those found normally in every eye may be a sign of retinal disease. These pathological scotomata can be any shape or size. If they occur in the periphery of the field of vision, they are more likely to be ignored or to go unnoticed. Diseases that cause this include multiple sclerosis and tumors on the pituitary gland.