After Napoleon forces Snowball off the farm, he uses Snowball's image to provide a scapegoat for the other animals. He blames all of the farm's problems on Snowball. This sort of scapegoating is a very typical tactic used by dictators to control their people.
This tactic allows dictators like Napoleon to control their people in two ways. First, it unites the animals behind Napoleon in a sort of "us vs. them" dynamic. They all feel like they are in it together against Snowball and so they should obey Napoleon. Second, Napoleon can use this as a weapon against any dissenters. Anyone who disagrees with Napoleon or his ideas can be branded as a follower of Snowball. No animal would want that because it would mean they were an enemy of the farm. They'd probably get killed for it and would at least be ostracized by their friends.
In these ways, Napoleon uses the idea/image of Snowball to control the other animals.
Snowball is like a scapegoat. Napoleon blames Snowball for all the misfortunes and bad things happening on the farm. He uses Snowball to control the other animals.