Some background information is important to answer this question.
Running a farm proves to be more difficult than the animal think. For one thing, their supplies begin to run low. For this reason, Napoleon, to the surprise to all to all the other animals, announces that he will work with humans. Mr. Whymper will help him obtain the supplies that the needs to run the farm. From this point, the animals start trading with humans.
When the animals are confused, the silver-tougued, Squealer, reminds the animals that there was no rule in animalism that prohibited the working with humans. In fact, he goes one step forward by saying that if they think that this new development is wrong, then they have been influenced by the traitor, Snowball.
In light of this point, it is clear that humans are back for two reasons. First, the animals need humans to have a successful farm. Second and more importantly, humans prove to be good trade partners. As the animals begin to produce, the pigs can now sell them to humans. All of this proves to be profitable, and in the end profits matter most for Napoleon and the other pigs.