The Dubon Magid zt”l (Sefer Ohel Yaakov) explains why a metzorah must be brought to a kohen as follows:
A metzorah sinned with his mouth, by speaking badly of others. The reason people do not guard their speech is because they fail to recognize how much influence their words can have both in this world and in the Upper Worlds. Hashem teaches them this lesson by sending a nega of tzora’as. Such a nega only is deemed tamei if it is declared so through words – if the kohen says that it is tamei, then it is tamei; if he says it is tahor, then it is tahor. The status of the nega hinges solely on the kohen’s words, and not on any knowledge. Chazal go so far as to say that even if a kohen who is an ignorant, unlearned person says that it is tamei and a Yisroel who is a talmid chochom says that it is tahor, the rule is that it is tamei, because all that matters is the kohen’s words.
Therefore, the Torah stresses that a man who is afflicted due to engaging in forbidden speech should be brought to the kohen as a lesson about the power of words.
