Cao Cao understands something that the others fail to grasp: Even though the Han Dynasty has little real power, the perception of imperial power among the people remains very strong, because at the end of the day everyone understands that they are subjects of the dynasty. That's what drives Cao Cao's decision to come to the aid of the exiled emperor: now that Cao Cao has amassed tremendous power and an enormous army, all he needs to become the most powerful man in China is the perception of authority. Allying himself with the emperor will not only endear him to the people, it will make it easier for him to manipulate the other lords into fighting one another, thus ridding Cao Cao of enemies and clearing the way for him to eventually inherit the throne.