First, equal rights and duties for all persons is fundamental to a just life. There really can not be a just society if some people are allowed rights that others are not entitled to. Next, there must be a concept like original position that includes impartiality with the veil of ignorance and is contracted because people are seeking to establish a mutually beneficial system, not create a new social system. In a perfect world, people would all be socially equal. No one wishes for people to be impoverished, poverty stricken, or on the last step of the class system.
Every person wants to do well in life.
In fact, they want those who succeed them, namely their children and family, to do as well if not better than they do. For this desire to be true, one must assume that society's view of social class is important to the average person. You would not want your child to do better than you unless you perceived the next level above yours was indeed better. Original position assures that every person has equal rights including the most basic liberty. Also, it mandates that social and economic inequalities are set up so that they are to be to everyone's advantage and so that the positions are open to all.