Moses speaks clearly in chapter 18 of the responsibility of a prophet to speak for God and the consequences that come if that doesn't happen (namely, death). What an incredible responsibility! The same responsibility is true for a pastor when he speaks in front of the congregation. Essentially, he is speaking for God: he becomes God's mouthpiece. If the words that are spoken do not line up with God's Word in the Bible, then the person is not truly a mouthpiece but a false teacher/prophet. How can a person know the difference? By knowing the Bible. Israel's kings were to study the scripture so they did not stray away from God's will; the same must be true for us.
Yes, the future king was to study the scripture, the copy he made with his own hand. He was to make his own copy to have all the days of his life. Would we know God’s word better if we each had to write it out for ourselves?