There exist pages for books that are "presumed" to exist based on author expectations, but have not been even conceived of yet, much less written, edited, titled, covered, published and read to be reviewed.
For example, Brandon Sanderson thinks that his current big hit series (Stormlight) will have ~10 books, he's currently half way writing the 5th book, so the 10th is a LONG way coming.
It's entirely possible he will change his plans, after all he did so for his other series Mistborn, he put four more books in between book 3 and what was originally going to be book 4 (now possibly book 8), if he doesn't change the plan again, or cancel the book series entirely and re-write the entire multiverse setup he has in his brain.
Yet a page for this hypothetical Stormlight book 10 exist:
https://www.goodreads.com/book/show/17250985-untitled
There is no harm in pre-emptively creating a page bookmarking purposes, I supposed, but it already has 22 written reviews and 94 ratings giving it 4.68 stars.
WHY? why allow such things for non-existent books? How hard is it lock star ratings and comment box until the book is, you know, actually existant and read?