to refill muscle glycogen. The theory as I understand it is that almost all carbs you eat at that point will be used to refill muscle glycogen and not go to fat. Some also say it has a super compensation effect, that your muscle will actually load with more glycogen than they normally would, but I am not sure about that one. The main reason seems to be the fat burning effect during the week, than refilling muscle glycogen so that workouts can still be productive .