Orwell writes persuasively in Animal Farm through the use of an omniscient narrator. The reader knows what some of the animals are thinking, and realizes that they are being taken advantage of much sooner than they do.
For example, consider this passage from chapter 6.
Once again the animals were conscious of a vague uneasiness. Never to have any dealings with human beings, never to engage in trade, never to make use of money--had not these been among the earliest resolutions passed at that first triumphant Meeting after Jones was expelled? All the animals remembered passing such resolutions: or at least they thought that they remembered it. (chapter 6)
The animals are questioning what they remember and don’t remember, but the reader knows that they are right- the amendments are being changed. By allowing us into the different animals’ heads like this, we are able to get a better grasp of what happened and why. We understand how the animals are manipulated.
If the story were told from third person limited and focused on one character, we would not necessarily know if the character was reliable as a narrator. With an omniscient narrator, we know what is going on is really happening. It is not just one character's perspective.