Though his vocation before the apocalypse is never mentioned specifically, we can assume that Master was probably an engineer with expertise in running power-generating facilities like Underworld. He might also have been a civil engineer that planned communities and basic services involved like water, electricity, sanitation, trade services, etc, so he was very valuable, possessing a certain type of brilliance in this arena. Because of his protective partner, Blaster, Auntie was never able to fully exploit him and had grown sick of his arrogance and opportunistic attitude, so she waited until she found someone who could dispose of Blaster: Max. Max, who's very savvy, as we all are aware, also saw the potential in Master to help build a new civilization for Walker's children. Max, who slowly regains his compassion and humanity through the series, probably believed that Master shouldn't be a slave to Auntie in Bartertown, so he decided to free him.