Orc slave master comes once in a while in orc villages. It gets special training that allows him to boost slaves production in captured castles and villages of Men and Dwarves. 
Slaves have multiple uses for the Dark side. First of all they can be sold in sea ports or forced to work in gold mines to boost their income. Alternately they can be sacrificed in dark portals to receive some mana. 
Slaves tend to run away when moved to their final destination, so some escort is needed to look over them. And slave master can handle a whole army of slaves all by himself!