I can see in the OEGB cock birds and cockerels they look quite different, but say a cockerel Orpington or a cockerel Langshan vs. a cockbird. As well as Cornish.
cbbantams, that's just the thing isn't it. The cockbirds (generally) are better than the cockerels. So, if you can fake a bird being younger than it actually is, you are able to move it to the top of the pickings in the cockerel class because the bird is more mature, better filled out and overall a better bird than the rest of the cockerels in the class. And by receiving a first place in the cockerel class, you are immediately able to have your bird in the selection for Best and Reserve of Variety.