Why the Ring?

When Tolkien was asked by his publisher to write a sequel to 'The Hobbit', he needed a link... so this is what he said about it himself :

“If you wanted to go on from the end of The Hobbit I think the ring would be your inevitable choice as the link. If then you wanted a large tale, the Ring would at once acquire a capital letter; and the Dark Lord would immediately appear.  As he did unasked, on the hearth at Bag End as soon as I came to that point.  So the essential Quest started at once.

But I met a lot of things on the way that astonished me.  Tom Bombadil I knew already; but I had never been to Bree.  Strider sitting in the corner at the inn was a shock, and I had no more idea who he was than had Frodo.  The Mines of Moria had been a mere name; and of Lothlórien no word had reached my mortal ears till I came there.  Far away I knew there were the Horse-lords on the confines of an ancient Kingdom of Men, but Fangorn Forest was an unforeseen adventure.  I had never heard of the House of Eorl nor of the Stewards of Gondor.  Most disquieting of all, Saruman had never been revealed to me, and I was as mystified as Frodo at Gandalf's failure to appear on September 22.  I knew nothing of the Palantiri, though the moment the Orthanc-stone was cast from the window, I recognised it, and knew the meaning of the 'rhyme of lore' that had been running in my mind: "seven stars and seven stones and one white tree".  The rhymes and names will crop up; but they do not always explain themselves... "

The Letters of J.R.R. Tolkien
Letter 163 – to W.H. Auden, page 216-7