The sheep represent the unthinking masses, those who will go along with any perceived authority as long as they are individually protected by their mob mentality. This is very common; people will often take stronger stances and act in way they would never consider as individuals. With the mob mentality, the sheep provide a loud and unthinking support for Napoleon to convince the other animals. Because the sheep never consider the bigger picture and never actually think about issues, they are easily used to disrupt and diminish Snowball's speeches:
...Napoleon was better at canvassing support for himself in between times. He was especially successful with the sheep. Of late the sheep had taken to bleating 'Four legs good, two legs bad' both in and out of season, and they often interrupted the Meeting with this. It was noticed that they were especially liable to break into 'Four legs good, two legs bad' at crucial moments in Snowball's speeches.
(Orwell, Animal Farm, msxnet.org)
Just like protesters at political events, the sheep create a general feeling of distrust for Snowball, the speaker. It is hard to explain and focus on issues when a large group is shouting their talking points. The other animals would normally be convinced by Snowball's speech, but they can't focus on the content, instead only hearing the refrain "Four legs good, two legs bad." Since they agree with this sentiment, the animals think that since so many animals (in reality, only the sheep) are shouting this, they must be disagreeing with Snowball. By diverting attention from the actual issues and focusing entirely on a meaningless emotional argument, Napoleon gains larger support.