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. Starting with different basic liberties is unfair. Another critical point in my just system is Rawls' idea of the difference principle. The purpose is to assure that everyone benefits. When the natural lottery distributes assets, it is via an arbitrary method.