What is Orwell saying by including this event?

The Soviet population became terrified of execution and internment in forced labor camps called Gulags. In the novel, the animals’ immediate response to the purge is fear and disillusionment. Shaken, Clover and the other animals try to take comfort in “Beasts of England”—they know that something has gone terribly, terribly wrong but cannot quite describe what or how. They want to focus on the positive ideas of freedom and abundance. Squealer shatters even that comfort when he announces that the song is obsolete and therefore forbidden. We can assume that the real reason Napoleon abolishes it is that, since the animals have committed it to memory, he cannot revise it like the Seven Commandments. Therefore, he forces the animals to forget it, along with the tenets of their beloved Animalism, to be replaced with a new song and new values that are looking more and more like the values under which Mr. Jones ran the farm.