Who can explain why there is both Old and New Biblical Testaments? What is a Testament, or Covenant?
There is a book in the New Testament which explains the answers to both questions, along with the necessity of Jesus Christ in the fulfillment of both Testaments, and the purposes of the Levitical Priesthood [the lower], and the Melchisedec Priesthood [the higher], of which Jesus is the high priest, and the ultimate purpose of Jesus Christ, "For it is not possible that the blood of bulls and of goats should take away sins."
Follow that last quote as a thread to find the book. Read and study it, and the answers to the above questions are offered to the studious. You need no further commentary.