The animals need a middle man as a go-between for contact between the farm and the outside (human) world. Despite the animals' efforts, the farm is not altogether self-sufficient, and from time to tîme products are needed from the outside (such as a dynamo for the windmill). Whymper is mentioned as fulfilling this function at least for a time, but as the story progresses it becomes obvious that the pigs seek human companionship for other reasons than just necessity. They strike up friendly relations with their neighbours, and if not yet "friends," they seem to nevertheless enjoy their companionship.
The word 'broker' implies a middle man for handling money and financial transactions. There is an exchange of money (bills) for timber later on in the story, but as it turns out the notes are found to be counterfeit. So much for honour among thieves!