Proceeding from the assumption that repeating something enhances both the effect and the importance of the statement, the repetition of "nevermore" reiterates the grief, despair and fatalistic hopelessness of the speaker in the poem.
I am from a school of literary criticism that doesn't concern itself much with the "intended effect" of an author. However, the mournful refrain of nevermore seems to serve both literary purposes (it is a device in the rhyme scheme) within the structure of the poem, as well as reiterate the central themes of the poem: alienation, grief, despair, hopelessness and ultimately, death.
