What is the purpose of dreams, altered perceptions of reality, and visions in otherwise realist fiction?

This is a pretty big question for this short space.

Let me comment on The Yellow Wallpaper by Charlotte Perkins Gilman. The narrator suffers from postpartum depression, a condition not understood in the 1800’s. For this reason, the story exhibits many of the elements of literary realism. The cultural norms at the time are reflected in the patriarchal need to control and pacify women when they show strong emotions. By choosing to write in the first person, the reader gets a realistic account of her mental illness spiralling out of control. The reader gets full access to the deterioration of the narrator’s mind as she fights to get out of her literal and metaphorical prison.