Bombadil connects the story to the future, not to the past. He is the only character whose fate remains enigmatic after the book concludes. He transcends the plot and is unaffected by the ring, unlike literally every other character, including gods. In cross-reference to his characteristics depicted in The Adventures of Tom Bombadil, and to the letters of Tolkien (which contain a specific denial of Bombadil being "Iluvatar", that is, Tolkien's fictional creator) I came to the conclusion years ago that Tom Bombadil is J.R.R. Tolkien inserting himself into the story. It's quite possible this is an unintentional self-insertion, and he's certainly not an author surrogate (which is a different and much more literal class of self-appearance, like William in The Naked Lunch), but nevertheless, the cap fits.
