The same answer anone would have for why they do anything. There's an obvious benefit from it. Now whether your idea of slaves and mine are the same or not is highly debated

"There ought to be a term that would designate those who actually follow the teachings of Jesus, since the word 'Christian' has been largely divorced from those teachings, and so polluted by fundamentalists that it has come to connote their polar opposite: intolerance, vindictive hatred, and bigotry." -- Philip Stater, Huffington Post
always working on cleaning my windows- me regarding Johari
always working on cleaning my windows- me regarding Johari