It is used to tell the narrator that his love is dead. It is also used for repetition.
It is the only word that the Raven knows how to say.
the answer might be "power".
Well, if you mean the poem that Edgar Allen Poe wrote, the theme was definitally horror. Edgar was just like that. It's rather strange, isn't it?
"Nevermore", perhaps? It's not clear exactly what you're asking about, but in the E.A. Poe poem "The Raven," "Nevermore" is the raven's response to all the narrator's statements or queries.
Raven Squad Bro Trailer
Raven Squad Gameplay Trailer
Drew Dellinger's Poem: Hieroglyphic Stairway
Drew Dellinger's Poem: Hymn to the Sacred Body of the Universe