It was an artifact of the way The Lord of the Rings began as a sequel to The Hobbit.
And what more can Hobbits do? They can be comic, but their comedy is suburban unless it is set against things more elemental.
JRRT, Letter 19, 16 December 1937
If you look at The Return of the Shadow, part 1 of Christopher Tolkien's history of how the LoTR was written, you'll see that
- Tolkien's publishers, Unwin and Allen, wanted a sequel to what they considered a successful children's book.
- such a sequel required some things left over from The Hobbit, and of course hobbits.
- Tolkien didn't consider The Hobbit as important a work as his larger mythology, and so that mythology was bound to creep in.
In The Hobbit, the Ring was just a magical ring that could turn you invisible. But there was a mystery about it: Where did it come from? How did Gollum get it? So Tolkien chose the Ring as a hook to build the story from. In hindsight:
The magic ring was the one obvious thing in The Hobbit that could be connected with my mythology. To be the burden of a large story it had to be of supreme importance. I then linked it with the (originally) quite casual reference to the Necromancer, whose function was hardly more than to provide a reason for Gandalf going away and leaving Bilbo and the Dwarves to fend for themselves, which was necessary for the tale.
Letter 257, 16 July 1964, quoted in the preface of the The Return of the Shadow
Since this was the One Ring to Rule Them All, there had to be other rings to rule. In one of the early drafts of the chapter that became "the Shadow of the Past" in the final LoTR, Gandalf tells Bingo Bolger-Baggins:
In the very ancient days the Ring-lord made many of these Rings: and sent them out through the world to ensnare people.
