The reason for this prohibition is widely disputed. Some commentators suggest that intoxicating drink was used in Canaanite religious practice. As representatives of the Lord, the priests were to be set apart. Others suggest, due to preceding context, that the prohibition must be connected to the disobedience of Nabad and Abihu. That is to say Nadab and Abihu may have been under alcoholic influence when they offered unauthorized fire before the Lord. In the end, the most likely reason is that intoxicating drink causes a priest to err in the performance of his duties as will be spelled out.
9 “Drink no wine or strong drink, you or your sons with you, when you go into the tent of meeting, lest you die. It shall be a statute forever throughout your generations.