Why was slave labor so important to the southern economy

“well,” he says, “there’s excuse for picks and letting-on, in a case like this; if it warn’t so, i wouldn’t approve of it, nor i wouldn’t stand by and see the rules broke—because right is right, and wrong is wrong, and a body ain’t got no business doing wrong when he ain’t ignorant and knows better. it might answer for you to dig jim out with a pick, without any letting-on, because you don’t know no better; but it wouldn’t for me, because i do know better.” the meaning in this excerpt is