It's better because the bread needs to soak up the custard. If the bread isn't stale, it contains moisture in it already, so there won't be any room for it to soak up the liquid around it as it bakes. If you're unable to stale your bread in advance, you can place cubes of bread spaced out on a large cookie sheet, and heat them in the oven at 300°F for about 15 minutes, or until dried out and slightly toasted.