In Animal Farm, what is the significance of Old Major's dream, and its significant to the development of the story?

Old Major's dream was about the state of happiness that will exist once Man is eliminated. In the dream, a tune his mother and the other sows sang to him in his childhood returned to him, and new words accompanied the tune. Old Major is sure that he has, in his dream life, uncovered an old animal anthem that has lain dormant for generations. It is called “Beasts of England,” and he sings it to the other animals. Orwell describes the song as “a stirring tune, something between Clementine and La Cucaracha” (32). The song glorifies the freedom and joy that will follow “Tyrant Man’s” overthrow, and he urges all animals to “toil for freedom’s sake,” even if they die before the cause is won.