Unless you plan to read the entire J.R. Ward Black Dagger Brotherhood series, there's certainly no point in reading this guide. You also don't have to read the guide to follow along with the series.
However, if you want a snippet of the way I wish that the book about Zsadist and Bella had been written, the novella about them contained in this guide provides exactly that. I would have been so, so happy with the third book if the writing had been done in this style. And, if you're a fan of the series, you definitely don't want to miss reading the novella, regardless of how you felt about the third book.
If you're a loyal follower of the series, of which Ward has plenty, you'll probably want the guide for your collection, in general.
My personal rating of The Black Dagger Brotherhood: An Insider's Guide by J.R. Ward is a B.