To teach — To direct the priest when to pronounce a person or house clean or unclean. So it was not left to the priests power or will, but they were tied to plain rules, such as the people might discern no less than the priest.
Related Commentaries of Leviticus 14:57
Leviticus 14:57
57 To teach when it is unclean, and when it is clean: this is the law of leprosy.