Although Animal Farm, as is well-known, is a political allegory, Orwell's tone is decidedly non-polemical. Think how much less effective this withering satire on Stalinist Russia would have been had Orwell tried to insinuate his very heartfelt political opinions into the text. Also bear in mind Animal Farm's famous subtitle: "a fairy story." Fairy stories tend to have a message, but the moral of the story is always more effective if implied rather than spelled out in an obvious manner.
It's not surprising, then, that Orwell avails himself of the third person omniscient point of view. He can do all kinds of things with it: he can describe, suggest, hint, insinuate, gently persuade. All of these political and literary objectives can be achieved by way of the greater detachment that the third person omniscient point of view brings. Orwell wants us to agree with him, but he also wants us to get there by thinking for ourselves, reaching our own conclusions on the basis of the details he provides.
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