This answer contains spoilers… (view spoiler)[I'm a little late on this one but I can give my take on it. Court was the one that told them to do so and they were the ones who turned in Hax as a traitor. So, Court could have wanted them to be up close and personal with the man they had sent to his death by making them scatter the bread. Kind of in a "we are responsible for this" feeling for the kids (even though Hax was a traitor). To be gunslingers they needed to be OK with the idea of death and causing death. Also, since Hax was a traitor the bread could have attracted birds quicker and made Hax get chomped up by the birds a little quicker. Kind of like a last FU even after death. (hide spoiler)]
Andrew I *think* the purpose of the bread was to prevent the birds from picking at the body... for a little while at least. I believe Cort meant it as a symbolic gesture, and indeed, when the boys look around and see just how many carrion-feeders have flocked to the hanging, they understand just how functionally pointless it is.