At the end. Mainly the pigs and the dogs became richer. In orwell's mind, why is this fact important?

If you're talking about the book, "Animal Farm" by George Orwell, it is symbolical. When Orwell wrote the book, he was talking of the Russian's communist society. That is what the whole thing represented. The pigs and dogs represented the leaders in the society. The fact they became richer was important because it represents what happens in a communist society. Everyone else becomes poorer and the leaders become richer.