Rawls addressed the question in moral philosophy of why we must keep our promises at various places in his work. Over the course of his intellectual career he defended different accounts of promising, which reflect broader shifts in his theoretical views. But they all share the feature of being “practice based” or “conventionalist”: they rely on the existence of a social practice of promising as an essential part of the explanation for why you should keep your promises.

The social practice of promising is the existence of a general adherence to the rule “Do A whenever you utter the words ‘I promise to do A.’” The existence of this practice is in the public interest. Because of this general adherence, it is possible for us to rely on people to A when they say that they promise to do so. And being able to rely on them in this way is very useful. When I know what other people are going to do I am in a better position to make my own plans.

The practice of promising also makes us better positioned to form stable cooperative relationships. Suppose that there is a project which, if completed, will benefit two different people but will only be completed if they both contribute to it. Neither person will be motivated to contribute to this project unless they can rely on the other person to do their part. By making promises to each other to do their part each person can rely on the other and thus the project can be completed, to their mutual benefit (TJ 304–305).