The simplest answer to that question is the obvious one: military history is the record of humanity’s many wars, including their preparation, execution, aftermath and the ways they affect the societies involved. We study military history for the same reasons we study other kinds of history: to understand how we got where we are and perhaps also to learn how to avoid repeating previous mistakes. Like all other aspects of history, our understanding of past wars evolves as new information comes to light and new interpretations replace the old.
A more nuanced answer to the question has to do with unexpected results. In any conflict, from the smallest skirmish through pitched battles and campaigns to whole wars, we generally expect the larger force to win. Yet history is filled with examples of the stronger side failing to subdue the smaller, or even falling victim to it.
How can that be? The answer to that requires a detailed inquiry into not merely what happened, but how it happened. That in turn leads to the analysis of the mechanisms of war: weaponry, organization, leadership, doctrine, the approaches used by each side to try to create combat power, as well as the choice of targets against which that power is to be directed. That all comes together at the intersection of intelligence, time, space, and force: in a word, strategy.