When you are talking about slaves and creativity, im imaging the sims, and slave-artists in basement (someone remember?) forcing to makes paintings all day long. Also, slaves doesnt means stupid, but strong, or uncreative. Slaves means slaves, they can be masters of arts, that are working like a slave, without payment. And also in stellaris, slaves CAN have "intelligent" trait without being "strong", dont they? And what that means, that they are less competent than "stupid", and "strong" but free pops?
It's largely a gameplay segregation to allow specialisation between ethics. Egalitarians are more inclined towards specialised resource production while Authoritarians and Xenophobes are inclined towards raw resources. It means that different ethics play differently, which is something this game sorely needs, more than being able to do everything with one ethic.
It also helps that Authoritarians don't require as much in the way of Consumer Goods thanks to Stratified Economy while Egalitarians have a few living standards that increase specialised resource production at the cost of higher Consumer Goods use.