Cooking with bran

Bran, also known as miller's bran, is the hard outer layers of cereal grain like oats, wheat, rice, rye, and others. It's rich in protein, iron, fiber, carbohydrates, fatty acids and other nutrients, such as B-vitamins.
It consists of the combined aleurone and pericarp. Along with germ, it is an integral part of whole grains, and is often produced as a byproduct of milling in the production of refined grains. Wikipedia

Nutritionally speaking, separating the germ and bran from the grain before processing it into flour is not a good idea. However, The main reason is the fact that the germ and the bran contain fatty acids which can go rancid. Therefore, flour made from grain that has had the bran and germ removed will have a longer shelf life.