What is the importance of the "other world" in Unfinished Tales: The Lost Lore of Middle-earth?

This other world contains three separate areas of interest, all which serve as settings for separate parts of the stories. The Far West is not well described in the narrative, but it seems to be where the Valar reside and where the Elves originally came from. Numenor is much closer to Middle-earth, but still apart, as it is an island. It is made up of a central part and five peninsulas that make it look it a little like a star. Numenor is described as being a gift to the people who come there. The Men who go there are very long-lived, especially the royalty. The island later sinks into the sea. Middle-earth, itself, is the dwelling place of many types of people and creatures. In different places, there dwell Men, Elves, Dwarves, Orcs, and Hobbits, among others. Middle-earth is the rising place of two separate evils, Morgoth and Sauron.