It is impossible for us to know what Jesus thought of the answer given by his disciples to the probing question he asked after his teaching about the kingdom (the question is found in Matthew 13:51). They affirmed that they had understood all that he had taught in the seven parables recorded in the chapter.
Whatever their grasp of his teaching was at that time he proceeded to illustrate for them their future role in his kingdom. He said that they would be like a wealthy man who could provide surprising and satisfying provisions for his guests when they visited his home. We can imagine a guest being surprised and delighted by the food on the table, the comfort of where they socialised, and the value of gifts that the owner would provide. The role of the disciples would be as if the wealthy owner took money from his treasury and gave it to those who were his guests.
How were they to do so? By teaching others what they had found out about the kingdom. Their teaching would be heavenly treasure, and it would be twofold in that some of it would be old and some of it would be new. Maybe the 'old' describes what they already knew about the kingdom from what had been said in the Old Testament and the 'new' describes what would yet be included in the New Testament. Perhaps the 'new' would also include being able to see the Old Testament in ways that they could not yet understand, but which they would see after Jesus was resurrected and ascended to heaven.
What does this have to do with us? We have access to the same treasury - the riches described in the Bible. Pastors and preachers often find things old and new in passages that they have chosen to explain to congregations. New here does not mean no one else has never seen it, but it is new for them. And such discoveries of kingdom realities are full of spiritual encouragement for themselves and others.
Yet we can also say that seeing old and new should be the experience of all Christians as they read the Bible. There should be fresh discoveries of the wonders of the kingdom of God. This should happen because we are reading a book that is all about Jesus and his kingdom. Because that is the case, we should be reminded of old things and should discover new things to share with one another as we travel together sharing the life of the kingdom.