This is called diabetic macular edema. It makes reading and driving difficult. If there are many areas of poor blood supply in the retina the eye tries to compensate by growing in new blood vessels. This is called proliferative diabetic retinopathy. We are born with just so many blood vessels, and they are supposed to last throughout our life.
New vessels coming in, at any age, are weak-walled.
Blood in front of the retina gives an appearance somewhat like fog. While the actual amount of blood is very small, when it clouds the vitreous in front of the retina or lies on the retinal surface, it prevents light from getting to the seeing elements of the retina. How severely vision is affected depends on the amount and location of the blood. If it lies over the center of the retina, the macula, reading or driving may be impossible. If the retina behind the blood does not receive the light well because of the diabetes injuries described above, this compounds the problem.