Everything must be in the book for a reason. If assorted bones were randomly added to a skeletal structure, it would not work. The same with a good story. It doesn't have leads that go nowhere or mysteries that go unanswered. Just as the skeleton is a framework for the body, plot is the scaffolding of a story.
Characters are the muscle and blood of any story.
Without them, you just have bare bones. Good characters, well drawn and interesting, attract readers into the story. A novel that's heavy on plot but light on character development can't grip readers the way a thriller must. Good characters in peril give readers a sense of personal investment and commitment. Setting and description are the skin and features of your book.