Image: Django Unchained (film)
The only way for slavery to truly work (to benefit the owners) is to get the slaves to police each other. That way the owner is free to carry on with their own affairs without having to spend the day controlling others. To this end a manager is required, one that can be completely trusted to represent and protect the interests of the owner. Interestingly, this mirrors the situation in the business world.
(Of course, slavery is morally wrong and ultimately works to the detriment of all. However, in many ways it is similar to what is called "wage slavery" today and similarly works to benefit a few from the labor of many.)