The Uruks are building Sauron's War Machine on the backs of conquered and enslaved humans. A burgeoning slave trade exists throughout Mordor, with newly captured slaves sent to Thaurband, and then shipped out via Great Beasts.
Throughout Mordor slaves are working in foundries and mills, growing food for the Uruks, and suffering on poles as object lessons in obedience. A slave's life consists of toil and beatings. This is a new Mordor, and human slaves are making it a reality. But the Gravewalker has brought a small glimmer of hope back into their lives.