People who think cultural relativism is plausible might find this view attractive because it explains the social nature of our moral obligations and why certain obligations hold rather than others. Moreover, those who think ethics is objective will find the idealization of social arrangements (what a contractor would ideally make) allows all cultures to be praised and critiqued (or rebelled against) in light of the ideal. The parallel to law is informative as well -- just laws are to be obeyed because they are sufficiently ideal, to be disobeyed because they undermine the ideal, and those who disobey good laws implicitly reject the cooperative agreement and can be punished with exclusion from the benefits of society (to return to the state of nature).