"Horse bread also allowed professional bakers to turn their leftovers into a commodity. [...] The most sought after form of bread was white bread, which required that a baker sift the whole-grain, stone-ground wheat they received from the miller, removing the grain’s hard outer layer, called “bran,” from the white flour. After turning the white flour into expensive white bread, bakers could recycle their leftover bran to make horse bread"

Makes a lot of economic sense - what they don't mention is that horses can get energy from fibre, while people can't, so this is biologically efficient as well as economically.