In the Summerset climax, you need a divine artifact in order to stop Nocturnal's plan to remake the world in her own image. After Sotha Sil punts on explaining why the supposedly "divine" tribunal isn't giving you anything in front of Mephala, and the artifact you do find has been rendered useless, you use Dawnbreaker in its stead and save the world.
But wait, isn't Dawnbreaker a daedric artifact?
Technically, its both divine (Aedric) and daedric because Meridia is both one of the Magna-ge and a Daedric Prince. https://en.m.uesp.net/wiki/Lore:Exegesis_of_Merid-Nunda