As other reviewers have made clear, this is most assuredly not a book for Tolkien neophytes. Therefore I shall assume the prospective buyer has a basic knowledge of the Middle Earth saga.
Unfinished Tales is indeed "the one truly essential set of supplementary/outtake material", and Tolkien scholars are strongly advised to pick this up as soon as they finish reading The Silmarillion. For two reasons:
1. "The Sil" is hard work - its presentational style, half-Bible/half-history-textbook, renders it inaccessible to a lot of people. But if you manage to finish it you can reward yourself with Unfinished Tales, which deepens your enjoyment of "the Sil" by providing more detailed (more gripping, more compulsively re-readable!) accounts of the same events, even though they are fragmentary and at-variance-with-other-writings.
The first section of the book begins with the expanded account of Tuor's early life and his mission to Gondolin which, for some, is the greatest of all Tolkien's obscure writings. But the piece that follows it, "Narn i hin Hurin" (tale of the children of Hurin), is certainly another candidate for the title - an extensive recounting of the disaster-ridden lives of Turin and Nienor. Even with a large section of the story (including the whole of Turin's sojourn in Nargothrond) missing, it winds up being the most emotionally draining thing Tolkien ever wrote.
The third section gives a more detailed background to the events at the end of the Third Age (i.e The Lord Of The Rings). There are accounts of "The Disaster of the Gladden Fields" and of the past tribulations of Rohan, and its special relationship with Gondor. There is Gandalf's perspective on the background to "The Quest of Erebor" (i.e The Hobbit), and, perhaps of most interest, Saruman's "Hunt for the Ring", or how lucky Frodo and Sam were even to get out of Hobbiton and begin their quest.
The fourth section contains almost all the existing data on the origin of the Palantiri, the histories of the Druedain (aka the woses) and the Istari (aka the wizards).
And what of the second section? Well, for one thing, it collects assorted writings on the subject of Galadriel and Celeborn - Tolkien's view of them continually shifting, a congruent history never quite emerging, even though he fills in a few gaps in the history of the Third Age in the process. The second section also fleshes out the history of the island of Numenor. Which brings us to...
2. Most of these posthumously published "archaelogical" volumes contain at least one "revelation" - a complete one-off in amongst all the spot-the-difference first and second drafts. And in this instance it's the the tale of "Aldarion and Erendis". It interrupts a capsule history of Numenor (a description of the island and a brief history of its ruling dynasty), shifting the focus from affairs of state to affairs of the heart, specifically the doomed romance between the sixth king of Numenor and a woman from the lower classes (so to speak: her shorter life-expectancy becomes an issue here). Where to begin describing this great tale? Well, if I may step out of character and oversimplify, Aldarion and Erendis began like Tristan and Isolde and ended like Charles and Diana. Which means two things become apparent here: Tolkien's flair for romance and his deficient social politics. In the end, whether Tolkien intended this or not, it works as a parable against arranged marriages.
Summary: Unfinished, but definitely not Unnecessary.