"And beginning with Moses and all the Prophets,
he interpreted to them in all the Scriptures the things concerning himself."
Why do we need the Old Testament? Because it is how we are to understand and interpret Jesus. Jesus himself states that all the OT is about him. The OT tells us of our need for a Savior and God's covenant promise to provide a savior. Therefore, we can't understand who Jesus is without understanding who he claimed to be. He claimed to be God's promised Messiah.