This final commandment summarizes one of the methods that Napoleon's regime uses to maintain power and control. Language manipulation, or language control, helps to manipulate the animals on the farm and keep them believing that things are better under Napoleon than they were under Mr. Jones.
In Animal Farm...Orwell examines the use of language and the subversion of the meaning of words by showing how the powerful manipulate words for their own benefit.
Many examples of language manipulation appear in the book, none more potent or obviously corrupt than the final commandment: "All animals are equal, but some animals are more equal than others."
This twisted language does not make logical sense. The term "equal" takes on a new meaning close to the way we define the term "valuable". We might also replace the term equal with a more political term appropriate to the implied meaning of the commandment and re-write the line: All animals are endowed with rights and privileges, but some animals are endowed with more rights and privileges than others."
By changing the meaning of simple words like "equal", Napoleon's regime takes control of the language in order to manipulate and control the animals on the farm.