Tolkien himself could have cut them- and no-one would have missed what is essentially a self-contained unit- but chose not to- so he must have valued them- along with the suggestion they bring into the book that there's a whole world elsewhere- and one that plays by other rules. Tom and Goldberry may not be able to cross the boundaries of their limited demesne but within in it they're gods. They live and stand apart from Middle Earth- and the Ring has no power over them- the only characters in the whole trilogy of whom this is true. Tom holds it up to look at it and Frodo is startled by the vision of his blue eye sparkling within the circle of gold. He then makes it disappear and appear again- like a street magician. For everyone else it's an object of fear, awe and warped desire; for Tom it's just a trinket.
Who are Tom and Goldberry? We never find out. They could be Nature spirits- genii loci- (and Tolkien I believe- in a letter- once spoke of Tom as the spirit of the Berkshire and Oxfordshire countryside) but no mere genius in any mythology possesses the power they do. Tom says he's old- older than history- and why should we disbelieve him? Their relationship appears symbiotic- with Tom ranging across his woods and downland while Goldberry stays at home, tethering him to his centre. He can't linger too long, he tells the Hobbits as he escorts them off his territory, because "Goldberry is waiting". Is she the source of his power- the sun round which he revolves? Could be.
Actually I don't think Tolkien himself knew quite what they were. They came to him from somewhere ouside the mythos he was so carefully creating- odd, anomalous, irresistible- as independent of their creator as they are of the ring he'd dreamed up.
I know some readers find Tom and Goldberry irritating. I don't. These chapters give us many of the trilogy's indelible images- the wicked old willow man, Tom dancing about in his ridiculous proto-hippy clothes, Goldberry among her water lilies, the hobbits laid out asleep in the barrow with the sword across their throats and the skeletal hand creeping round the corner. Everything that Tolkien has to offer- comedy, terror, mysticism, landscape- are here in the highest concentration. For me they're the heart of the book.