During Robin Hoods time men were sent off to fight in the crusades, an on-going effort by Christians to retake the Holy Land from Muslims. The robots have a similar quest. Like the half-faced man in Deep Breath they seek the Promised Land. What kind of heaven do robots dream of? Why do they need the hope of something better than what they have? The Doctor accuses Robin of providing false hope. He dismisses him as a myth the peasants need to believe in order to give them a reason to persevere through hardship and despair. When Robin argues he is not just talking about his physical existence but his power as an inspiring heroic figure of myth and legend. It doesnt matter if people believe he is a real person, it is his story that motivates people to be braver, stronger, better than what they thought they could be.