Aaron and his successors as high priest were given control over the Urim and Thummim by which the will of God could be determined. God commissioned the Aaronide priests to distinguish the holy from the common and the clean from the unclean, and to teach the divine laws (the Torah) to the Israelites.
Aaron and his successors as high priest were granted authority over the Urim and Thummim, which enabled them to discern God's will.  God tasked the Aaronide priests with distinguishing between holy and profane, and teaching the divine laws (the Torah) to the Israelites.