In medicine, case histories have long been used. When you go to a medical doctor for the first time, you may have to fill out a case history. You may answer a form that asks questions about your health and the health of your immediate family. Why do doctors ask you these questions?
The answer is that those questions may lead to important clues in your diagnosis. For example, questions about travel overseas and about your consumption of untreated drinking water may prove very useful in the diagnosis and treatment of gastrointestinal discomfort. Answering yes to both of those questions opens the door for diagnoses related amebiasis, or the harboring of amebas in your gut. You may have ingested those amebas from well water, or from poorly treated water overseas. However, answering no to those questions makes such a diagnosis much less likely.
In clinical psychology, case histories are routinely used when patients are admitted to treatment facilities. The questions will be different, for the most part, from medical case histories, in that the questions will be more about behavior, not about health. Questions may be asked about family structure and conflicts, about significant life events, and about ways of dealing with people and problems. Diagnoses and treatment suggestions will come, partly, from the clinical case history.
In both of these examples, the idea that the past predicts the future is implicit. In forensic psychology the same relationship between past and future is seen. For example, a parole board may have to decide whether or not to parole a convict based on that convict's behavior in prison. But, what they are really deciding is whether or not they believe that convict will no longer engage in the behavior that caused the original conviction. Similarly, expert witnesses may testify in court as to the mental state of a defendant at the time a crime was committed. That testimony is usually based on a case history collected by the expert witness before the trial.
Case studies are also used in business and economics to study how one company or industry profited from certain conditions while others did not. So, this method has a great deal of use in a variety of settings.