This is actually a good question, and the answer is that the book gives people who are new to this kind of group something to talk about and relate to. Such a b

This is actually a good question, and the answer is that the book gives people who are new to this kind of group something to talk about and relate to. Such a book group has two anchors: "that group of friends I see each month", and "that book that's interesting (or fodder for derision)". Newcomers to the group don't have that first anchor, but having the second allows them to participate and feel comfortable.