Of the three songs (this one, "May It Be" and "Gollum's Song") that were written for the end credits of the three movies, this is the only one that made it into the movie proper.
For his final summation and the climax of the entire story, Shore chooses to ignore all the great music we have heard stated and restated all this time, and instead give us music that is completely new except for two strategically-placed foreshadowings. The dramatic impact of this is huge; Shore is telling us that we have arrived, not merely at the end of the story, but at a special place we have not seen before — the place the whole story has been leading us to all along.
There are other places where I have pointed out resemblances between Shore's music and other people's (see Chase and Shire, for instance), and for the most part I consider those the kind of coincidences that tend to pop out of the unconscious of a person like Shore whose musical knowledge is vast. In this case, however, I maintain that Shore is here deliberately and consciously honoring Wagner, who ended his gigantic 4-opera cycle Der Ring des Nibelungen with a theme of unearthly beauty that he had foreshadowed only once. To me, this ending feels very much like that one: uplifting, autumnal, full of loss, and very moving indeed.
If you're still not convinced, see Shore's parting shot.