When someone loses a person he loves dearly, the thought inevitably occurs that he will never see that person again throughout eternity, either in the flesh or in the hereafter. When King Lear is talking to his dead daughter Cordelia, he says:

Thou'lt come no more,
Never, never, never, never, never!    (5.3)

And when the pitiful, mad Ophelia is mourning her father in Hamlet, she recites the following verse:

And will he not come again?
And will he not come again?
No, no, he is dead;
Go to thy deathbed;
He never will come again.  (4.5)

In Poe's "The Raven," the black bird perches on the bust of the Goddess of Wisdom, symbolizing that the one word it keeps repeating is the incontrovertible truth which there is no negating and no escaping. When the poem ends with the speaker sitting in utter dejection under the bird's shadow, it is that single word and the thought it expresses that prolongs his hopeless mourning.

The Raven is probably one of Poe's most famous works. The word Nevermore in the poem is one that leaves the reader feeling sadness. Poe uses this word the create the tone for the whole poem. 

The whole poem is set around a man grieving for the love of Lenore. We see that he is in such despair. He tries the read to get his mind off of losing her, only to have a Raven come and talk to him. With the Raven only saying Nevermore, again and again, we see unending despair in the man. The Raven, itself, represents the despair the man feels. The man is in such sadness, that the repeating words of the Raven, echo his sadness throughout the whole poem. 

"From my book surcease of sorrow- sorrow for the lost Lenore- For the rare and radiant maiden whom the angels name Lenore."

The loss of his love, Lenore, is almost too much for the man to bear. The Raven, seems the reiterate that fact by saying Nevermore. Poe had such a clever way of making words come alive. He was truly a master at his craft. The Raven is just one piece of work that he shows us his mastery. 

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.