Control of access to information is important in any dictatorship. In the beginning, efforts are made to teach all of the animals how to read and write so that they can be more equal. In time, the pigs move away from this idea because it is in their best interest that only the pigs can read and write. Literacy is key to participating in government, and by keeping most of the animals illiterate the pigs managed to control them.
Propaganda is key to maintaining a totalitarian regime. As long as the animals only had the information the pigs wanted them to have, they would remain subdued. The commandments are a perfect example of this. At first, they are displayed for everyone to see. They are idealistic and promote the idea of equality. However as time goes on the pigs adapt them to their own interests. Since fewer and fewer of the animals are literate, the pigs can do this.