Snowball, characterized as a traitor, shows how dictators use scapegoats to ramp up fear and keep control.
This is most clearly shown when it says, "It seemed to them as though Snowball were some kind of invisible influence, pervading the air about them and menacing them with all kinds of dangers." The dogs growling and showing their teeth every time the name "Snowball" is mentioned strikes fear into the hearts of the other animals. Making Snowball out to be a villainous traitor, creates an atmosphere of fear and helps Napoleon to stay in control.