If the mass is broken up by red lights then the traffic will be slowed significantly more than if the mass stuck together and rolled on by in just a couple minutes.
Sticking together is also for safety. When large groups start breaking apart people get confused and do really stupid things in an attempt to catch up with the main mass. Plus, when the group breaks apart and slows down traffic even more then drivers get very angry and try to pull stunts to be on their way. These stunts are usually pretty reckless and dangerous to the cyclists.