Napoleon realizes early on that his power is going to be dependent on the ability to enforce it. Napoleon does not speak much, and does not offer much in way of rallying the other animals around him, like Snowball. Yet, he understands a long term vision of maintaining control resides in the very nature of establishing control and a presence. This happens through his taking the puppies. Napoleon wants a group of dogs that will be submissive only to him, cut off from any attachment to the other animals. This would include their own mothers. Napoleon instructs them in the manner that he wants. He teaches them in the way he wants to. He has this small army that will be responsive only to him and what he desires. Napoleon recognizes early on that power belongs to the individual who can enforce his will on the body politic and through taking the puppies, Napoleon envisions this possibility. It is a long term approach, a rationale that make it evident that Napoleon is about power and control and everything else is a distant second.
They are meant to be Napoleon's personal army, and are meant to uphold his leadership by force, and crush any opposition towards him.
He trains them to be his vicious and loyal protectors, in order to control the farm. He is able to achieve total tyranny because of the fear they inflict upon the rest of the animals. They are representative of the KGB during the Russian Revolution.