Why did Elijah and Moses appear with Jesus on the mountain?

And there appeared before them Elijah and Moses, who were talking with Jesus. Mark 9:4

            Many believed, from a Malachi prophecy, that Elijah literally must appear on earth one day.

“See, I will send the prophet Elijah to you before that great and dreadful day of the Lord comes. Malachi 4:5

            Jesus said that this Elijah, a prophet, was John the Baptist.  But, just to clear up any confusion, the real Elijah showed up too on a mountain with Jesus during his transfiguration.  Moses also appeared. 

            These two Old Testament figures highlight two aspects of Jesus—lawgiver and miracle worker.  They appear in submission to Jesus, at his right and left.  This moment revealed Jesus’ supremacy to any other hero in the Old Testament.