As per blood cholesterol measuring, levels of blood glucose are measured in mmol/l or mg/dl. Blood glucose testing is important in determining if someone suffers from Hyperglycemia (high blood glucose/ sugar) or Hypoglycemia (low blood sugar), and in some cases Diabetes. If you're already aware of one of the aforementioned problems, regular blood testing can help track progress/ management. It's also advisable during certain dietary conditions, such as fasting or low carb diets.
As regards Diabetes, the closer blood glucose is kept to normal, the less likely someone's going to suffer Diabetes complications. As mentioned above, regular blood glucose tests can help identify whether meal plans and exercise (& possibly medication) are working to keep blood glucose levels as close to normal as possible.
Blood glucose ranges will vary during the day (around meal times) and, therefore, so will associated blood glucose target levels. The person conducting the test (hopefully a medical professional) will take this into account (along with several other factors like diet), for example, a generally recognised blood glucose target range for a non-Diabetic person two hours after a meal is under 140 mg/dl, compared to under 160 mg/dl for a Diabetes sufferer. Whereas, if the test was conducted upon waking (before breakfast), target levels would be roughly 30 mg/dl lower (Under 110 mg/dl for non-Diabetic, 90 - 130 mg/dl for someone with Diabetes).
For further information on Diabetes, please vist the International Diabetes Federation (IDF) or the National Diabetes Information Clearinghouse (NDIC).