Answer: to reveal that dictators manipulate others and use deception to further their ambitions.
Step-by-step explanation:
Orwell's main purpose in this passage is to reveal that dictators manipulate others and use deception to further their ambitions.
Despite the famine and the food situation, Napoleon was well aware of the bad results that might follow if the real facts of the food situation were known, and therefore he decided to make use of Mr. Whymper to spread a contrary impression. This showed that he deceived others.