Oral tradition was unreliable, so Scripture was often written to preserve it for the future as a memorial (Exodus 17:14) or a witness (Deuteronomy 31:24-26). that it might be there for the time to come (Isaiah 30:8). The Book of Law was lost, after all, during the reigns of Manasseh and Amon. When Hilkiah rediscovered it, its teaching came as a great shock, for it had been forgotten. (2 Kings 22-23; 2 Chronicles 34)
The permanent and abiding form of God's message was not its spoken form but its written form, which explains the need for Old Testament canon.
(The Origin of the Bible: Newly Updated by F. F. Bruce, J. I. Packer, Philip W. Comfort, and Carl F. H. Henry, 2020. The Canon of the Old Testament by R. T. Beckwith. Pages 53-54.)