Albert Barnes Commentary Leviticus 10:9-11

"Drink no wine nor strong drink, thou, nor thy sons with thee, when ye go into the tent of meeting, that ye die not: it shall be a statute for ever throughout your generations: and that ye may make a distinction between the holy and the common, and between the unclean and the clean; and that ye may teach the children of Israel all the statutes which Jehovah hath spoken unto them by Moses." — Leviticus 10:9-11 (ASV)

When the priest was on duty, he was to abstain from wine and strong drink, so that he would not commit excess and thus become disqualified for carrying out the precepts of the ceremonial Law (Leviticus 10:9).

Strong drink: The Hebrew word is used here to denote strong drinks of any kind except wine made from the grape (Leviticus 10:10).

Unholy ... unclean: These terms refer to what is common (that is, not consecrated) and what would cause defilement by being touched or eaten . (Leviticus 10:11)

That is, “that you may, by your example in your priestly service, preserve the minds of the Israelites from confusion regarding the distinctions made by the divine Law.”