Merely pointing to the fact that some people have a lot more than others is less compelling as a critique; it invites the response So what? Those who have more aren't hurting anybody; you're just appealing to envy. By contrast, being able to show that those who enjoy a higher socioeconomic status have to a considerable extent achieved and maintained that status by forcibly expropriating and oppressing the less affluent provides for a far more effective indictment.
p. 425 - "Left-libertarianism, market anarchism, class conflict and historical theories of distributive justice"