Quote from: TimThe (in essence) landed gentry encouraged racism as a means of obtaining yeoman farmer support for institutionalized slavery (the very thing keeping the small farmer from getting his piece of the pie): "Well, we're not rich and our lot in life ain't never going to improve, and I can barely keep my family fed, but at least we're not sub-human livestock like THEM."

It has always been helpful to identify a THEM to point away from yourself with.

Cheap labor has been a driving force throughout history. Slavery is one of the less creative ways of obtaining it. "Cost effective workforce" I believe is the modern term for it.
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While I put slavery into most of my settings, when it actually makes its appearance in games (rather than just being background element) I tend to use it to do two things:

1) Reinforce the brutality of the setting. My own homebrew settings tend to be set in periods of social crisis or change, and enslavement reveals the brutality of the kinds of power politics that PCs would otherwise glibly indulge in. I go for the grand guignol aspect of politics in RPGs anyhow, and slavery is a key part of that (along with public torture and execution of one's foes, etc.)

2) Reinforce the distance between the players' mindsets and the characters'. I'm not really fond of games where the characters are mediaeval pastiches, but the players give them 20th century attitudes. I'm not an "Immersionist" (whatever that means) but I like to have characters navigate the world in ways that are sensible for the time and place they live in. Very often there are clashes there between how you or I would react to a certain situation, and how a person with a mediaeval mindset (even if it is a pastiche) would react to it. Slavery is one such test, and can be useful to help players see the difference between how they feel on a topic and how their characters would.

Coincidentally, family structures are the other big one I fiddle with to achieve this effect.