Christ and His disciples were, for once, on the eastern side of the Sea of Galilee, a non-Jewish area (the Decapolis). Part one of the story - the demoniac was waiting for Him, for Legion knew who God was. Faced with this divine power, Legion fell down and asked for mercy (not forgiveness), confessing Jesus as God's Son, but not his Lord. Isn't it sad that demons have more knowledge of God than does the average Christian? But they do not have love for Him, only fear.
Satan kept breaking earthly chains and making his possession destroy other earthly objects (rocks, clothes). Legion begged not to be cast into hell right away, to be free to roam the earth (albeit leashed). So Jesus transferred them to pigs, unclean animals Legion subsequently destroyed.
Part two: shocked herdsmen (compare/contrast with the Christmas situation) told the happening. The now-ex-demoniac was clothed (white robes of righteousness...) and in his right mind (chasing after God). So are we after Baptism. Let us not, as the Gentiles there, look at this miracle with fear, rejection, and unbelief. Let us remain cleansed, as followers of Christ, telling in our homes and spheres what He has done for us. Go, clean, sane, and clothed.
Soli Deo gloria.