Answer: It was the basis for their entire economy.
Explanation:
Unlike the North, which had less arable lands, the South was extremely fertile and therefore developed into an agricultural based economy. They farmed cotton and tobacco, which were both labor-intensive crops. Specifically, cotton plantations required pickers to walk the fields in the burning sun trailing sacks that would weigh up to 100 pounds when full. Their hands would end up bloody from the sharp spikes of the cotton plant. Even until the end of the Industrial Revolution (from the late 18th to mid 19th century), the economy of the South was still booming from Eli Whitney's cotton gin.
In short, the agricultural based economy was so profitable that the South saw no reason to industrialize. However, the crops that they farmed were extremely labor-intensive and they saw slaves as affordable labor. Racism was normal at the time, with the Vice President of the Confederation preaching "the great truth of racial inequality". All these factors combined led to the persistent use of slave labor in the South.