They're necessary, yes--but does it really help to read a book's summary before you read the book itself?
You have to know something general about a book to know whether you might or might not like it, or at least whether or not you might find it worth reading. So you generally must at least glance at the summary. But it can be quite often that I find myself not in need of the summaries--or wishing they weren't even there.
This is for different reasons.
1) The book is a sequel - If I like a book and it has sequels either already out or coming out, I will most likely read them because otherwise I feel like I haven't finished something. So I don't need the summary because I'm going to read the book, anyway, and looking at the summary might just spoil a surprise plot element.
2) The book is from an author I'm loyal to - I'm loyal to an author for different reasons. I watch a couple of YouTube authors, and so I feel loyalty to read all their new books just because I feel like it's my way of paying them back for their videos that I enjoy--and because it's curious to see what kind of books the people who make these videos are writing. Or maybe it's an author I've just really enjoyed and want to continue reading more of: these might either be the further works of a dead author or the newly published works of a living author. Either way, just the author's name has me interested.
3) The book comes under recommendation - Recommendations can be personal, or maybe a book is just a title you've been hearing a lot about and want to investigate for yourself. Like Wicked: I've heard a lot about it but I'm not sure I've ever actually read the summary and yet I'm beginning to think I might like to read it one of these days, just to see what people like so much about it.
4) The book is a story I already know - Mainly this covers books that have been made into movies, but can also be classics or other stories that are basic knowledge. I already know the story so, even though I haven't read it yet, I don't need to see a summary.
5) The book has other factors going for it - Sometimes you take time to browse a bookstore and you jut come across random things that look good. A title catches your eye, or a book cover--something about the tone that interests you. You can turn to the summary to learn more, or you can just flip to a few of the pages inside and read a couple of sentences, get a feel for the author's style. It's amazing how much more you can grasp sometimes by doing this instead of looking at the summary. Granted, you'll usually want to look at the summary, too, but not every time.
It's very freeing, in fact, to not read a book's summary beforehand. In cases 2-3, I usually at least glance at the summary (to make sure that it doesn't sound like something completely out of my scope of interest) when I buy the book--but then I sometimes purposely let the book sit on the shelf for long enough that I've half forgotten the summary. That way, when I finally pick it up, it will be new to me, every little detail of plot, character, and setting will be new and my reactions can come to me with the turning of each page.
Freeing, yes, that's what it is.