This bit of machinery is called a bolter and it's purpose was to separate the flour and bran once the wheat had been ground in the mill. These different grades would come down through the individual chutes as shown below.
The top grade [i.e. the much sought after white flour] was classified as the "fair" and this was for human consumption, the "middling" was rougher and used to feed the pigs and the really rough stuff left behind went for cattle feed. It was interesting to learn the origins of another everyday phrase at the weekend.