'Appointing' in the Bible

"The Lord answered, "I'm Jesus, whom you are persecuting. But get up and stand on your feet, because I've appeared to you for the very purpose of appointing you to be my servant and witness of what you've seen and of what I'll show you.

Now if perfection could have been attained through the Levitical priesthood for on this basis the people received the Law what further need would there be to speak of appointing another kind of priest according to the order of Melchizedek, not one according to the order of Aaron?