Nietzsche is worth quoting at some length on this important point. Second essay, Genealogy of Morals:
* Here one more word concerning the origin and purpose of punishment—two problems which are separate or should be separate. Unfortunately people normally throw them together into one. How do the previous genealogists of morality deal with this issue? Naively—the way they have always worked. They find some “purpose” or other for punishment, for example, revenge or deterrence, then in a simple way set this purpose at the beginning as the causa fiendi [creative cause] of punishment and—they’re finished. The “purpose in law,” however, is the very last idea we should use in the history of the emergence of law. It is much rather the case that for all forms of history there is no more important principle than that one which we reach with such difficulty but which we also really should reach—namely that what causes a particular thing to arise and the final utility of that thing, its actual use and arrangement in a system of purposes, are separate toto coelo [by all the heavens, i.e., absolutely] from each other, that something existing, which has somehow come to its present state, will again and again be interpreted by the higher power over it from a new perspective, appropriated in a new way, reorganized for and redirected to new uses, that all events in the organic world involve overpowering, acquiring mastery and that, in turn, all overpowering and acquiring mastery involve a new interpretation, a readjustment, in which the “sense” and “purpose” up to then must necessarily be obscured or entirely erased. No matter how well we have understood the usefulness of some physiological organ or other (or a legal institution, a social custom, a political practice, some style in the arts or in a religious cult), we have still not, in that process, grasped anything about its origin—no matter how uncomfortable and unpleasant this may sound in elderly ears. From time immemorial people have believed that in demonstrable purposes, in the usefulness of a thing, a form, or an institution, they could also understand the reason it came into existence—the eye as something made to see, the hand as something made to grasp. So people also imagined punishment as invented to punish. But all purposes, all uses, are only signs that a will to power has become master over something with less power and has stamped on it its own meaning of some function, and the entire history of a “thing,” an organ, a practice can by this process be seen as a continuing chain of signs of constantly new interpretations and adjustments, whose causes do not even need to be connected to each other—in some circumstances they rather follow and take over from each other by chance. Consequently, the “development” of a thing, a practice, or an organ has nothing to do with its progressus [progress] towards a single goal, even less is it the logical and shortest progressus reached with the least expenditure of power and resources—but rather the sequence of more or less profound, more or less mutually independent processes of overpowering which take place on that thing, together with the resistance which arises against that overpowering each time, the changes of form which have been attempted for the purpose of defence and reaction, as well as the results of successful counter-measures. *
Given this repeated usurpation of social practices for new purposes, knowing the "origin" of something tells us nothing about its current social functions. You need to analyze existing power relations to understand that.
Miles