Paul is in an actual legal fight now (probably preferable to the physical fights he has come from), and even before Felix he is still preaching. Here with all the players of his troubles, he brings together the Old and New Testaments to show once again how it all comes together in Jesus. That's a different approach than many might take, as choosing to fight for our own lives might take precedence over preaching. Not Paul, though. He sees his life with that one singular purpose, and showing the world Jesus is always on his agenda.