I’m working on a review of a book that is quite a ways on in a series—a series that I’ve never read from before. I already know from experience that some of the people who read my review will be very angry that I could consider myself able to review said book without having read the rest of the series, and who will tell me that all of my problems with the book wouldn’t be problems if I had just read those other books.
It got to be a pain trying to insert the explanation for why this can be useful into so many reviews. And I know it was annoying for anyone who reads multiple reviews on this site to have to read that over and over again. That’s why I’ve added my rant about series books as a permanent page on the blog.
tl;dr My rant comes down to this: there are some very good reasons why most series books should at least marginally be able to stand on their own. There are also some very good reasons why prospective readers might need that piece of information out of a review. If you want the specifics, follow that link and enjoy the long form!