When asked what makes the study of history a "discipline," academic historians usually respond (after finishing their drinks) that their books and articles don't just describe a particular event or time period - they also attempt to determine the enduring significance of that event or period. Bernard Bailyn, who dominated the Harvard History Department for nearly forty years, had a more succinct way of making this point: in his graduate seminars, he would frequently begin his criticism of a paper or article by asking "So what?"
Recently, a student asked Professor Timothy Burke (of Swarthmore) if historians have a stock answer to that question. The short answer to the student's question is "No," which generates much consternation in graduate seminars and considerable angst in late-career professionals. Burke very usefully sat down and thought up a longer answer to the question, a list of answers in fact, which he recently posted here.
Some of these items appear rather unhelpful on first glance; for instance, number 2 ("the past is not prologue") and number 10 ("the past is unknowable") suggest that history has nothing to teach the present. Actually, though, one of the most important historical monographs of the twentieth century, C. Vann Woodward's Strange Career of Jim Crow, falls squarely into category 2. By demonstrating that racial segregation was not an old practice in America, but rather was the product of (then) fairly recent laws, Woodward could argue that American racial attitudes were not deeply rooted but were created by political and legal decisions that could be reversed. Woodward's argument was debatable, but it proved highly persuasive: the NAACP entered an early version of the book as evidence in the case of Brown vs. Board of Education (1954), and Martin Luther King, Jr. called the book "the historical bible" of the civil rights movement." Would that we could all write something as useful.