At first it seems obvious; audiences need to understand the story, both in the concrete sense of “what is actually happening”, and the more abstract level of ideas, themes, and overall meaning. “What is the story about?” “What is the story trying to say” These are important questions to consider when editing a story.
A good mystery is all about the question. If the protagonist is a criminal then the question is “how will they accomplish the difficult task”. If the protagonist is a detective then the question becomes “how did they do it”. In both cases the beginning establishes how difficult the task is or was, then spends the rest of the story gradually dropping hints and clues, giving audiences the chance to try and solve the mystery. The key is to carefully manage information so that audiences feel like they have a chance, but don’t solve the mystery before the end of the story. Many mysteries use comical side plots to break up what can be a dry main story.
Commercial fiction is written to entertain, but audiences can be very picky about the type of story they want to read or watch. Most genres engage the same issues as literary and mainstream, but they engage the issues within the genre framework, disguising a familiar topic in the trappings of their genre.
To a writer, every story is unique, but words like “unique”, “good”, and “interesting” are almost meaningless by themselves. People need a frame of reference. Publishers need to know what kind of story they’re reading in order to evaluate it properly, and audiences need some way of deciding whether this is the right story for them.