The demonstrations have different purposes for different people. By that I mean, for Orwell they are a way to comment on Soviet history: that nation held stage-managed "spontaneous" activities to create the illusion that their population supported their politics. For the animals, there is another purpose, or rather, several. In Chapter IX you'll see that things have gone pretty badly for the animals before the demonstrations start. These are meant to distract the animals from the cold and hunger. They are meant to rally support behind Napoleon, and to raise their spirits—and to show Napoleon's control of the situation.