The unnamed narrator of "The Yellow Wallpaper" is an "unreliable narrator" because of the warped perspective she places over the story. It is not apparent at the outset because there is no frame of reference for the reader in the story style, which is written in the form of journal entries. As the narrative continues however, it becomes more and more clear that the narrator has, at best, a tenuous grip on reality. She starts to personify the wallpaper, convinced that it can have real effects; trapping people inside, being her friend, and even harming her child. Upon reading these strange visions, it becomes unclear to the reader if anything she writes is the truth or simply a psychosis-induced delusion. The ending is especially unclear, as the narrator refuses to explicitly state what she had just accomplished. She seems to have escaped her room, getting past John in the process, perhaps even killing him. Of course, in her mental state, it could just as easily have been a fabrication scribbled in a journal.
This leads the reader to question the writing: Why use an unreliable narrator as storyteller? The answer lies in the assumption that characterization is more important than storyline. The reader can with their own imagination, infer the various minutia of the plot. The importance is placed on showing the reader the depths of the narrator's mind, allowing them to experience the feeling of seeing through that person's eyes.