The animals were necessary, because they allow Orwell to disguise his chauvanism and elitism.

In Animal Farm, the pigs are biologically more intelligent than the other animals. It’s not simply a matter of lack of education, in fact, there is a literacy campaign, it just fails because the workers are stupid and lack the capacity to learn:

As for the pigs, they could already read and write perfectly. The dogs learned to read fairly well, but were not interested in reading anything except the Seven Commandments. Muriel, the goat, could read somewhat better than the dogs, and sometimes used to read to the others in the evenings from scraps of newspaper which she found on the rubbish heap. Benjamin could read as well as any pig, but never exercised his faculty. So far as he knew, he said, there was nothing worth reading. Clover learnt the whole alphabet, but could not put words together. Boxer could not get beyond the letter D. He would trace out A, B, C, D, in the dust with his great hoof, and then would stand staring at the letters with his ears back, sometimes shaking his forelock, trying with all his might to remember what came next and never succeeding.

This innate lack of intelligence means that the animals representing the working class can never (even theoretically) develop into a political entity, capable of asserting political positions. Far from being able to develop and propose solutions, the animals can barely comprehend political questions:

According to Napoleon, what the animals must do was to procure firearms and train themselves in the use of them. According to Snowball, they must send out more and more pigeons and stir up rebellions among the animals on the other farms. The one argued that if they could not defend themselves, they were bound to be conquered; the other argued that if rebellions happened everywhere they would have no need to defend themselves. The animals listened first to Napoleon, then to Snowball, and could not make up their minds which was right; indeed, they always found themselves in agreement with the one who was speaking at the moment.

This means that true liberation and self-governance of the animals is impossible and any attempt at such a goal will only lead to the establishment of a new ruling caste.

In this way, Animal Farm is not merely anti-Stalin or anti-authortarian (in fact, it presents authoritarianism as an unavoidable necessity), but rather anti-revolution and anti-liberation. Which is probably why it’s taught so widely and treated like gospel in capitalist countries.