Why do recipes with whole wheat flour often contain all purpose to?

Whole wheat flour contains all three parts of the wheat kernel including the bran, germ, and endosperm. Although this retains wheat's natural nutrients and high fiber content, it also produces denser baked goods. To lighten up whole wheat baked goods and ensure an acceptable texture, many baking recipes will include both whole wheat flour and refined, more finely milled all-purpose flour.