They are useful as a supplement to literature, especially if the copy of the book you’re reading is a bare bones editions without any footnotes. They’re useful for a historical background and keeping track of characters in novels which have a lot of them, and some books have more than one name for each character to add to the confusion.
In my local bookstores it looks like SparkNotes has taken over. Same general idea - plot summary plus some commentary. I can’t remember the last time I saw a hard copy of a CliffsNotes in the wild.
I was amused a few years ago to see the reverse happen. My son’s 2nd grade class had a project to put together an issue of the school newspaper. Students volunteered for their article topics. One kid did a movie review of the then current Percy Jackson movie. Now, the movie diverged quite a bit from the book. It was obvious the kid just recycled a book report and dropped in the names of the actors from imdb. He even mentioned characters and events that weren’t in the movie.