They actually played a pretty important role in early civilizations. For one, building a major urban civilization takes a lot of work. Slaves could be used for building projects to enlarge the city and its defenses, for agricultural labor to ensure that the people of the city could eat, or as domestic assistants to kings, lords, and shop-owners.
Slavery viewpoints played a major role in the Democratic Party split. Slavery in itself is for of injustice where one overpowers the other. It emphasizes that there's definitely a split among people.
