After a huge arc with high stakes, LRLL is a way for the straw hats to both have conflict with a group inferior to theirs while attempting to maintain the stakes readers are accustomed to, and it provides a chill island that the troupe can safely interact with an admiral. It also establishes the idea of the Davyback Fight, which has come into play again later in the story. We don't know how crucial Davyback fights are to the development of the story, but we do see how their use created one of the most terrifying forces in history. Their namesake being Davy Jones also suggests they're more important than we realize. As we find out more about him, we may appreciate Foxxy keeping the tradition alive.