Animal Farm is a satire of the Russian Revolution, distilled down to a small venue that can be easily understood. Because the real-life people important to the Russian Revolution are represented by animals, the animals need to be able to both speak and present their ideas easily. Without these abilities, they would simply be moving around at random, as animals do. The entire book starts with a dream by Old Major; there is some scientific evidence that animals can dream, but their dreams are likely not similar to human dreams. This also speaks to the final moral of the story, that the pigs, who have taken over the farm and are exploiting the other animals, are no better than the humans they deposed:
What was it that had altered in the faces of the pigs? Clover's old dim eyes flitted from one face to another. Some of them had five chins, some had four, some had three. But what was it that seemed to be melting and changing?
(Orwell, Animal Farm, msxnet.org)
If the animals could not speak, the pigs could not communicate with humans to form their eventual alliance. By the same token, if the animals could not think as humans, they could not form their revolutionary ideas, and so the story would never start. It is not known if animals have the ability of abstract reasoning as humans do, nor if they dream in anything but colors or shapes, but both of these attributes are vital to the story.