I think that such a slave could have many uses. Woman slave could be used for breeding and speed up the process of recovering heard numbers after some catastrophe. Also, it could be skill based - where slaves with unique skills would be traded (like making ropes, sharp objects, bowls), something that would be hard to grasp for ancient people.
There is also a "mule" thing, where slaves would be used for carrying stuff. And of course the last - religious. Could be that group would have a "different looking" person in a heard as a charm/talisman (breeding with it would result in healthy children - thanks to genetic diversity).
But it could be nothing. More research is important for sure.
But it could be nothing. More research is important for sure.