This was because, during the Nara period, control based upon the Ritsuryo system (a system of centralized government based on the ritsuryo code) functioned effectively and, to extend the control based on the Ritsuryo system over every part of the country, it was necessary for the kokushi to move to the assigned province to maintain direct control.
奈良期はまだ、律令制による統治が有効に機能しており、律令制に基づく支配を地方まで貫徹するため、国司が任地へ赴き、現地支配を行う必要があったためである。