Vitamin B-12 is an essential nutrient that cannot by synthesized by animals or plants, only by micro-organisms. Most of B-12's benefits are related to it's co-enzyme activity, such as enzymes needed for the production of DNA during cell division. It is also necessary for the methylation of homocysteine, a harmful by-product of protein metabolism.
From the FDA's website: "As part of a well-balanced diet that is low in saturated fat and cholesterol, Vitamin B12 may reduce the risk of vascular disease. FDA evaluated the above claim and found that, while it is known that diets low in saturated fat and cholesterol reduce the risk of heart disease and other vascular diseases, the evidence in support of the above claim is inconclusive."