Why is the song of beast of England important to the animals in the beginning, and why do the abolish it at the end?

The song promises the animals emancipation from slavery, by humans, and a new dawn of animal autonomy. Napoleon basically says that there is no longer a need for the song as the rebellion is over. Really though, Napoleon no longer favours the words to the song. The song speaks to animal freedom and equality, Napoleon wants none of that. Napoleon wants absolute power.