Diabetes is a condition where a persons blood sugar is chronically too high. This happens due to insulin resistance in Type II diabetics and due to decreased production of insulin in Type I diabetics. Chronically increased blood sugar results in damage to blood vessels in the entire body. The eye is the only structure in the entire body where blood vessels can be visualized directly. This allows us to assess for vascular damage due to diabetes and high blood pressure directly.

The American Academy of Ophthalmology's recommendations on monitoring of diabetic patients is as follows:  
Type 1 diabetics should be examined 5 years after disease onset and atleast yearly after.
Type 2 diabetics should be examined at the time of diagnosis and atleast yearly after. 

The picture below illustrates diabetic damage which we monitor for during routine examinations.