The key to Rawls’ elaborate system is the intellectual device of the original position.
The OP is supposed to clarify issues of “social justice.” We imagine that people gather up for the discussion of what social institutions are best, but they forget, in order not to be swayed by their own personal interests, who they are in the actual society. They do not know their religion, social position, wealth and income, their particular ends, or anything else that might prejudice their judgment.
Under this veil of ignorance, they — or rather the philosopher who is shattered into numerous ghostly persons — seek to come to an agreement regarding the just basic structure of society.
I grant Rawls the assumption that the OP is a useful tool of reasoning. But the actual contract that will be produced in it is another thing entirely…