If you actually want to understand why Tom Bombadil is in the story, read Christopher Tolkien series on the creation of the Lord of the Rings. It was originall

If you actually want to understand why Tom Bombadil is in the story, read Christopher Tolkien series on the creation of the Lord of the Rings. It was originally envisioned as a sequel to the Hobbit with a slightly more grown up audience. When JRR wrote it, he hadn't even settled on Sauron as the bad guy. He vaguely thought the bad guy would be Treebeard! He got Frodo and co. all the way to Rivendell before he connected his other story (the Silmarillion) to the LOR story. Bombadil was left in, in my mind, because he had to explain how four hapless hobbits could throw the black riders off the trail and still escape from the forest. And he didn't really have any editorial constraints until the movies came along. Sure, you can invent some just-so-story to explain him but the truth is that the tale evolved in the decades it took to write and so it has vestigial parts.