They can be stored in facts. They can be held in variables. They can be compared for identity. Predicate values have no persistent representation.
Predicate values have many usages.
One of the most important is for callbacks . A callback is a predicate that is used to call back from some used entity to the user of this entity. Then this event source invokes the callback whenever an event occurs. Another very typical example is a Windows event handler. The window is however intended to work in many different situations, and therefore it cannot know which cursor to use in which parts of the window.