Answer: The Old Testament lays the foundation for the teachings and events found in the New Testament. The Bible is a progressive revelation. If you skip the first half of any good book and try to finish it, you will have a hard time understanding the characters, the plot, and the ending. In the same way, the New Testament is only completely understood when it is seen as a fulfillment of the events, characters, laws, sacrificial system, covenants, and promises of the Old Testament.
If we only had the New Testament, we would come to the gospels and not know why the Jews were looking for a Messiah (a Savior King). Without the Old Testament, we would not understand why this Messiah was coming (see Isaiah 53), and we would not have been able to identify Jesus of Nazareth as the Messiah through the many detailed prophecies that were given concerning Him, e.g., His birth place (Micah 5:2); His manner of death (Psalm 22, especially vv. 1,7-8, 14-18; Psalm 69:21, etc.), His resurrection (Psalm 16:10), and many more details of His ministry (Isaiah 52:19f.; 9:2, etc.). See more atthe website I have provided below.