Moshe details the rules that a Jewish king must follow, including the mandate to personally write two Torah scrolls and keep one with him at all times, as a reminder to remain humble and follow G-d’s law (17:18-19). Since a prerequisite for becoming king of the Jewish people was a higher level of fear of G-d, why was this directive included?
The Rambam (commentary on Avos 1) says that the appointment of a person to a high position may engender a sense of self-importance. To counteract this effect, possessing and reading daily from the Torah will help ground the king and keep him focused. This concept is relevant to everyone, not just kings. The Torah grounds us and frames our lives; daily exposure will enhance our experiences and better our future.