This story is an allegorical fable. Fables are not true but told to prove a point, thus Orwell took some creative license to build the story. When allegory is used, it is meant to represent on several levels something that is real. In this case, as the story is unfolding readers of that current time would have recalled the situation of the Russian Revolution which Orwell modeled his book after. For every generation since the original publication, it has been a sincere warning to watch that revoltions or modifications in government should work to always remember the people, as Old Major was trying to point out.
The abilities to speak and reason give Orwell the ability to create differing intellects among the animals so that a certain breed could take over and become the new government by the end of the story. If one group did not have ability to be better in some way than the others, the story would not have been able to prove the point by the end.