While Mark could well have begun his account of the public ministry with an example of Jesus' teaching—a parable, say, he chooses this dramatic action (accomplished, though, by a sharp word: “Quiet! Come out of him!”).
It is nearly impossible for us to know what diseases afflicted the people who came to Jesus for help. But the texts do reveal the social consequences of their affliction and how Jesus remedied those consequences.
Job’s reflections on the miseries of human life are meant to provide a background for the healing work of Christ, of which the gospel speaks. It is from such miseries as Job speaks of that Christ comes to save us.