People have a tendency to make light of the sin of loshon hara, said the Dubno Maggid. They say to themselves: ''What are mere words? I am not harming my friend in any way by simply speaking about him.''
The Torah therefore requires that the metzora be brought to the Kohen, in order for him to witness what man's speech is capable of doing. With one word, the Kohen defines the status of the metzora, making him either pure or impure - such is the power of man's words!
Source: Rabbi Yisrael Bronstein