Why do some boxers (Foreman, Liston, Benn, Holyfield) weight train their biceps?

Everyone is using some kind of weight resistance to train. It may not all be the standard free weight system or heavy weight, but they are very important. Whoever is telling you they don't lift when training is probably either lying, joking, or not a very good fighter. I even seen Floyd Jr. doing curls with weights. Weights build stamina, probably the most important thing for a fighter past the 8th round.