Due to the narrator’s confinement, she begins losing her sanity. Most importantly for this story, we see the narrator’s descent into madness through her eyes. Readers stay with the narrator as her mind grows more chaotic and as she begins seeing shapes in the wallpaper. This is the ultimate example of showing, not telling. We have to deduce from her frantic writing style that there isn’t actually a woman trapped in the wallpaper; the narrator just thinks there is because she’s losing her grip on reality.
In "The Yellow Wallpaper," the narrator’s rich imagination might have found a productive and healthy outlet in her writing, but being forced to repress her imagination instead leads her to madness.