Rawls believes that a key purpose of ethics is to explain our intuitions. This is to be done by devising a grand, overarching theory of morality which explains why certain behaviours are good while others are bad, and seeing how well the theory corresponds with the complete set of our intuitions.
Utilitarianism is one such system. Indeed, Rawls sees it as the only presently existing system. The problem is that there are a number of cases where utilitarianism is rather unintuitive. The usual response to this has been to reject the need for a unifying system, and instead go by pure intuitionism. Rawls, however, wants to create an entire edifice as an alternative to utilitarianism, which he believe does a better job of capturing our intuitions.
In order to create this alternative, he borrows ideas he likes from many places. He sees his role as being not so much as an originator of ideas, as a combiner and unifier of other people's ideas.
"The great utilitarians... were social theorists and economists of the first rank; and the moral doctrine they worked out was framed to meet the needs of their wider interests and to fit into a comprehensive scheme. Those who criticised them often did so on a much narrower front... They failed, I believe, to construct a workable and systematic moral conception to oppose it." (pp.xvii)
"An important test of a theory of justice is how well it introduces order and system into our considered judgements over a wide range of questions." (pp.xix)
Rawl's view that ethical theory should unify our intuitions is illuminating, but not uncontroversial. What it does mean, however, is that Rawls is staking his theory upon its conformity to intuitions in "a wide range of questions." It is important to note he says "a wide range", but not "all" questions.
Why might it be useful to have a unifying theory? There are two obvious answers. First, our intuitions do not cover every case. In such uncharted waters we will need a system of morality in order to work out right from wrong. Secondly, our intuitions are useful but they are not infallible. Centuries ago, slavery was seen as good and natural, for example.