When Alexius I Komnenoi implored the pope (Urban II) for help, he meant it to reconquer the lost Byzantine lands (and therefore return in to their de facto ruler, a.k.a., the Byzantines). Urban however, decided to merely use that as a façade in order to then invade the Muslims, and retake the holy land.
He also saw the wanted to unite Christianity as a whole (All the sects at the time, Orthodoxy, Catholicism, Paulicianism, Iconoclasts, etc. into a single conglomerate, fighting a common enemy.)
It made an agricultural industry in the south possible.
For those owning plantations, it made great wealth possible.
It set up a class system.
Even people with small farms seemed to have been able to afford 1 slave, so the south had economic reasons for engaging in the civil war. There were 69000 farms in the south when the war broke out. Each had at least 1 slave so they were very dependent on that labor.
C - English Bill of Rights x
