To Question the Importance of the Old Testament is to Question the Importance of Jesus Himself
Jesus believed the entire Old Testament to be Scripture.
Christians must never dismiss the importance of the Old Testament.
Jesus emphasized the importance of the entire Old Testament.
Do not think that I came to destroy the Law or the Prophets. I did not come to destroy but to fulfill. For assuredly, I say to you, till heaven and earth pass away, one jot or one tittle will by no means pass from the law till all is fulfilled. (Matthew 5:17-18)
Jesus told His audience that the Law and the Prophets (representing the entirety of the Old Testament) were so important that He would not change or alter even the smallest detail of it.
He came to fulfill the Old Testament. This is an important claim by the Lord.
All the law and all the prophets point to Him and will be fulfilled in Him down to the smallest detail. Everything that is in the law and the prophets culminates in Christ, and He is the fulfillment of them. It is the most stupendous claim that He ever made. - Lloyd-Jones, Studies in the Sermon on the Mount
After His resurrection, Jesus reminded His followers of this statement.
Then He said to them, "These are the words which I spoke to you while I was still with you, that all things must be fulfilled which were written in the Law of Moses and the Prophets and the Psalms concerning Me." And He opened their understanding, that they might comprehend the Scriptures. Then He said to them, "Thus it is written, and thus it was necessary for the Christ to suffer and to rise from the dead the third day, and that repentance and remission of sins should be preached in His name to all nations, beginning at Jerusalem.” (Luke 24:44-47)
Jesus called the entire Old Testament “the Scriptures,” and He reminded His disciples that it all pointed forward to Him.
The apostle Paul reminded the churches in Rome of the benefit of the Old Testament - because it points to Jesus.
We then who are strong ought to bear with the scruples of the weak, and not to please ourselves. Let each of us please his neighbor for his good, leading to edification. For even Christ did not please Himself; but as it is written, "THE REPROACHES OF THOSE WHO REPROACHED YOU FELL ON ME." For whatever things were written before were written for our learning, that we through the patience and comfort of the Scriptures might have hope. (Romans 15:1-4)
When we spend time reading from the Old Testament, we need to remember that we are reading the Word of God - and His Word points to His Son, Jesus Christ.