On a basic level, I think that Orwell includes Benjamin to bring to light how some individuals simply choose to remain distinct from politics. One of the conditions of the modern setting is that individuals have the ability to disengage from political reality. Benjamin is an example of this. He withdraws from the political fervor of the other animals, not really interested in the intense political change that envelops the farms and the other animals. When other voice their optimism for the potential of what can be on a political level, Benjamin withdraws with some cynical attitude that others fail to heed or to comprehend. Benjamin's only loyalty is to his friend Boxer, something that he thinks exists outside of the realm of political activity.
Orwell includes Benjamin to detail how some individuals refuse to involve themselves within political reality. Yet, Orwell also shows that while individuals have this conscious choice, in some form, politics envelops everyone. Benjamin deliberately chooses not to pursue a form of the political notion of the good. However, he is the first to realize what is going to happen to Boxer when he is able to read the sign on the truck taking Boxer away. He quickly acts, but it is too late. This helps to bring Orwell's primary point to light in that while individuals may choose to disengage and not involve themselves with what is happening in the world, they do so at their own peril. Benjamin realizes this too late. In many respects, Benjamin is included to discuss how those who retreat from politics end up making it easier for those in the position of power to do what they want in a manner that discards and disregards the voice of the people. Benjamin's inclusion in the text proves the saying that Ralph Nader was fond of mentioning in that "If you don't get turned on to politics, then politics will turn on you." I think Orwell would agree with this and his inclusion of Benjamin proves this idea in the story.