A promise is typically attempted when reference to reputation or prior action is insufficient to secure trust. Promises typically occur in situations characterized by doubt, uncertainty, and distrust.

A promise can boomerang on the promisor — that is the whole point of what Hume called this “artificial contrivance for the convenience and advantage of society” — and it takes the extensive mutual trust of the members of a society to keep this contrivance well serviced, with its coercive teeth in good shape. If its implicit threats are not credible, if we reinstate defaulting promisors after a brief and profitable formal penance in the bankruptcy courts, then we have ourselves to blame if promises lose their public force and if politicians and bank managers do not even bother to avoid giving lying promises. “Read my lips” has come to mean “Truster, beware.” 

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There are very many more or less trusted powerful persons who have given no formal promises to those dependent on them. It is the inept politician who makes rash and unnecessary promises, rather than herding his sheep by other, less dangerous means. 

- Annette Baier

Trust

The Tanner Lectures on Human Values 1991