...out?

I tend to examine the issue from the economic and technological perspective. Until the widespread adoption of electricity and steam power, nearly all physical work was performed through a biological mechanism which actuated a simple machine (wheel, lever, etc.). Now yes there were items such as water wheels and windmills but these were static. If you wanted a source of motive power that you could move around, it was primarily biological. Now this biological engine could either be something like a horse, mule, water buffalo or elephant or it could be a human being. The latter has certain advantages on that it is capable of a wider series of actions and is not necessarily buttonholed into a single role. Now to make use of this resource you can either entreat it to do so (provide pay and benefits) or you can compel it to do so which reduces it to the level of a smart ox. The latter course allows you to reduce your resource usage (just provide sustenance and shelter) while maximizing the return. And thus we have the institution of slavery and all the horrific things it entails.