Mo'ed Katan: To teach us that the Kohen is not permitted to examine Nega'im on Yom-Tov, irrespective of whether it is to declare him Tahor or to declare him Tamei. ¹

Sifra #1: To teach us that, jnst as it is a Mitzvah to declare a Metzora Tahor, so too, is it a Mitzvah to declare him Tamei, and that, jnst as it is a Mitzvah in Eretz Yisrael, so too, is it a Mitzvah in Chutz la'Aretz. ²

Sifra #2: To teach us that, although, initially, the same Kohen who declares the Mtzora Tahor declares him Tamei, ³ if he dies in the middle, a second Kohen may continue to examine him. ⁴