Why do people highlight stuff in books, or texts?

The play, doctor faustus, opens with a prologue. by describing faustus's beginnings as a child "base of stock" and his end as his "waxen wings" melted when "heaven conspired" to stop him, the chorus subtly calls to audience's minds, as they begin to view the play, the commonly held idea of the great chain of being the pact with the devil the seven deadly sins the renaissance man