What is Poe's intended effect/purpose through the repetition of "Nevermore" in The Raven?

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 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.