the "one true ring" is a plot device that brings characters together
The "one true ring" in the Hobbit brings all of the characters in the story together. It actually continues throughout the entire Lord of the Rings series. This desire to either attain or destroy the ring is what the entire plot centers around.
Other examples of a "MacGuffin" is the Holy Grail in the Arthurian Legends and the maltese falcon in The Maltese Falcon.
