Why Do Aliens Enslave Human Beings?

Are human women in any particular danger from alien barbarians? Yes, because the barbarians sell them -

"...at high prices to the human renegades and rebels."
-Poul Anderson, Rise Of The Terran Empire (New York, 2011), p. 333.

In fact, they are "...the luxury trade..." (ibid.).

Poul Anderson really was able to rationalize every cliche and absurdity of pulp magazine covers. Apart from women as such, human beings with qualifications like a degree in nuclear engineering are valued by aliens who have acquired space technology and weaponry without understanding them whereas untrained human beings merely do jobs that should be automated and do not last ten years, being easily replaced.

Why do some of us like fiction that combines futuristic technology and extraterrestrial environments with past social relationships like feudalism, imperialism and slavery? Whatever the reason, Poul Anderson knew how to make it work. Slavery appears twice in his History of Technic Civilization. When the Empire has expelled the barbarians, it reintroduces slavery as one part of its penal system.

I value Poul Anderson's works because he excels not only at this kind of space opera but also at serious speculative sf and sometimes even combines them.

"...Rome has been much abused. Lay off Rome for a while. And give me no spaceships in feudal settings...unless, of course, you are Poul Anderson, but you are most likely not."
-Greg Bear, "Tomorrow Through The Past" IN SFWA Bulletin, Fall 1979.