One benefit to federation in book-focused social media is it helps stop Amazon from buying the centralized services. They bought Goodreads and Shelfari. It's great to know they won't be buying the book-iverse.
What’s wrong with Amazon? They made it possible for self published writers to actually make a living. They disrupted the big five publishers and have done a lot for independent writers. Even the indy bookstores require an author to go through corporate publishing gatekeepers. Amazon gives literally any writer an opportunity to sell books.
Amazon is too big. It attracts too many bots and fake reviews. They also have no incentive to remove fake positive removes as it is a conflict of interest for them to sell as many books as possible.
I buy tons of books through Amazon and own a Kindle. That being said they're horrible at running book social networks. Of the two they bought they killed one and froze development on Goodreads. A federated book community will stop them from swooping in and killing the whole party.