Why is some all-purpose flour bleached, and when should I use it?

Most flour is chemically bleached with very small amounts of benzoyl peroxide or chlorine dioxide.  It bleaches the color more quickly than would happen naturally.  This protects the flavor since it is the small amount of fat in the flour, combined with air and time, that results in flavor change.  Flours for making bread-like products need to be able to form a strong elastic dough so they can entrap the carbon dioxide released by baking soda or baking powder.  They are often bleached in order to perform well.   Bleached flour works best for quick breads, muffins, pancakes, and waffles.   Unless it is frozen, using the product after the “use by” date is not recommended.  If frozen, flour will not become solid. It will retain its original powdery form because there is so little water present in it. 
 
Source(s):
nal.usda.gov.  “Search the USDA National Nutrient Database for Standard Reference.”
accessdata.fda.gov. “Everything Added to Food in the United States (EAFUS)”