The usefulness of a theory in psychology is determined by its ability to provide a comprehensive framework, predict future events accurately, and stimulate the discovery of new knowledge.
A good theory in psychology needs to do three things well:
Cover a Lot: It should explain a wide range of stuff about its topic.
Guess Right: It should be able to predict what will happen in the future pretty accurately.
Keep Us Learning: It should make us curious and want to learn more, leading to new discoveries.