14When you enter the land the Lord your God is giving you ... and you say, "Let us set a king over us like all the nations around us," 15be sure to appoint over you the king the Lord your God chooses.... 16The king, moreover, must not acquire great numbers of horses for himself.... 17He must not take many wives, or his heart will be led astray. He must not accumulate large amounts of silver and gold.
18When he takes the throne of his kingdom, he is to write for himself on a scroll a copy of this law... 19It is to be with him, and he is to read it all the days of his life so that he may learn to revere the Lord his God and follow carefully all the words of this law and these decrees 20and not consider himself better than his brothers and turn from the law to the right or to the left. Then he and his descendants will reign a long time over his kingdom in Israel.
from Deuteronomy 17:14-20
So what is this list of commands about? Why are they given? And so what? Here's my take on it. Kings have the same set of tendencies that the rest of us do, and these commands appear to be a set of prudent practices -- they are ways to avoid certain kinds of trouble. Especially that last one -- write a copy of the Law and read it all the days of his life.