While the milling process helps improve white flour's shelf life, it does remove nutrients like some dietary fiber, iron, B vitamins and micronutrients. Since 1941, however, fortification of white flour-based products with some of the nutrients lost in milling, like thiamin, riboflavin, niacin, and iron was mandated by the US government in response to the vast nutrient deficiencies seen in US military recruits at the start of World War II. This fortification led to nearly universal eradication of deficiency diseases in the US, such as pellagra and beriberi (deficiencies of niacin and thamin, respectively) and white bread continues to be an important source of these key vitamins to this day.
Folic acid is another nutrient that the government has mandated is added to enriched grains like white bread. In the US and Canada, these grains have been fortified with mandatory levels of folic acid since 1998 because of its important role in preventing birth defects. Since fortification began, the rate of neural tube defects has decreased by approximately one-third in the US, resulting in a cost savings of an estimated $33.7-125 million annually.
All taken from Wikipedia.