How to justify aliens wanting to have slaves?

While aliens taking slaves is an opld story. it is rather hard to justify. After all, if they can travel between star systems, why would they want to take slaves? Don't they have better technology to do everything slaves can do, and with smaller risk of rebellion?

I found one justification in Galactic Civilizations game series, in their Drengin Empire. The Drengin are fully aware that robots can work better than slaves. But slavery is part of their cutlure and they are not willing to let it go. They also say that they view the use of machines as "dishonorable". Not that is stops the fact that their closest (and, most of the time, ONLY) allies are sentient robots (the Yor). And the Drengin also literally taker pleasure in suffering of others (via telepathy of some sorts), so they are mostly sadists who use torture pof slaves as entertaiment. So thye Drengin have good reasons to want to take slaves.

But what do you think. Do you have any other explanations for aliens wanting to take slaves? Do you think the Drengin are explaining it well?