that ye may put difference between holy and unholy

And the Lord spake unto Aaron, saying,

Do not drink wine nor strong drink, thou, nor thy sons with thee, when ye go into the tabernacle of the congregation, lest ye die: it shall be a statute for ever throughout your generations:

And that ye may put difference between holy and unholy, and between unclean and clean;

And that ye may teach the children of Israel all the statutes which the Lord hath spoken unto them by the hand of Moses.

Leviticus 10:8-11

The LORD made a rule for the priesthood not to drink strong drink before attending or leading public meetings and ceremonies.  This was because it could impact their ability to differentiate between holy and unholy and clean and unclean.