Slavery

Once a civilization reaches a certain level of advancement, we start to see a significant fall off in population growth or even population decline. It's already started here on Earth, and is likely an even bigger problem in more advanced civilizations. As a civilization advances, the value of a large family which provides manual labor to support you in your old age is replaced by pure liability as the socio-economics shift away from the family farm to government and corporate reliance.

Children are becoming more expensive to raise as medical and educational needs increase, and the payoff has disappeared as people in advanced nations rely on Welfare and Investments to cover the cost of aging out.

Now imagine what a few more millennia of this could do to the population of an Earth like world. Technology requires a larger and larger consumer base to cover the ever expanding cost of more complex technologies, but your advanced civilization has a shrinking consumer base among its own people. This means in order to survive as a civilization, they need to import people from worlds with positive population growth.

So, the advanced civilization intentionally maintains Primitive Worlds where the populations will grow, and then trades goods with them as fair payment for human cargo. While these slaves will be forced to fill the bottom echelons of the advanced society, over time their descendants will be properly educated and integrated into the main society and new slaves will be brought in to refill these bottom tiers.

Slavery

Once a civilization reaches a certain level of advancement, we start to see a significant fall off in population growth or even population decline. It's already started here on Earth, and is likely an even bigger problem in more advanced civilizations. As a civilization advances, the value of a large family which provides manual labor to support you in your old age is replaced by pure liability as the socio-economics shift away from the family farm to government and corporate reliance, and the required level of education you need to even become a productive member of society able to support kids cuts more and more into how many years you actually can use to have kids.

Children are becoming more expensive to raise as medical and educational needs increase, and the payoff has disappeared as people in advanced nations rely on Welfare and Investments to cover the cost of aging out.

In highly developed parts of the world, the average person only has a 50% chance of choosing to have children, and those that do have an average of only 2.5 kids because they either wait so long to start that infertility becomes a major obstacle, or they choose to cut off for economic reasons. If everyone everywhere in the world followed this reproductive strategy, the human population would decline to less than 10 million by the year 2400.

Technology requires a larger and larger consumer base to cover the ever expanding cost of more complex technologies, but your advanced civilization has a rapidly shrinking consumer base among its own people. This means in order to survive as a civilization, they need to import people from worlds with positive population growth.

So, the advanced civilization intentionally maintains Primitive Worlds where the populations will grow, and then trades goods with them as fair payment for human cargo. While these slaves will be forced to fill the bottom echelons of the advanced society, over time their descendants will be properly educated and integrated into the main society and new slaves will be brought in to refill these bottom tiers.