Napoleon uses the sheep as propaganda spreaders and scapegoats.
The sheep represent the common people who did not have enough education to understand that they were being led around. They are uneducated and no one is able to teach them to read, and they can’t even learn the tenets of Animalism beyond “Four legs good, two legs bad.” Yet they are constantly used for misdirection and scapegoating. They interrupt meetings, confess to things they didn’t do, and basically allow Napoleon and his henchmen to get away with murder.
At first, the sheep are just used to spread the word in a simple, mass way. Four legs good, two legs bad. Later, Napoleon uses the sheep as misdirection. Consider an argument between Snowball and Napoleon in Chapter 5.
Then the sheep broke out into a tremendous bleating of "Four legs good, two legs bad!" which went on for nearly a quarter of an hour and put an end to any chance of discussion. (Chapter 5)
Napoleon balks, dogs growl, and the sheep put an end to the discussion. Later, such use of the sheep continue. They don’t even really realize how they are being used. They are just going along for the ride, because they are being manipulated by Squealer, Napoleon’s minister of propaganda. The same thing happens when a sheep confesses (although other animals confess too), and is executed.
When a person follows another person’s lead without thinking, the person is accused of being a sheep. Therefore it kind of makes sense that Orwell would choose the sheep as the animals that are the crowd, the followers, in this story. They do what they do because they don’t know any better. They get caught up in Napoleon’s fervor, and when one of them starts, the rest of them follow. What Napoleon wants, he gets.
In the Animal Farm, Napolean uses the sheep as a flattering tool to praise his administration and propaganda by making them bleat continuously in the appraisal of his deeds or works so as to silence the other animals from arising any further questions and to make them believe that he was always right.
The sheeps were made use by Napolean so easily because those sheeps were ignorant and stupid. They would continuously bleat in a row praising Napoleans propaganda without even realising what it really means... The sheeps were mainly used by Napolean for vocal appraisal due to their high pitch voice only.