Symbolism, Imagery, Allegory
Sybil, Connie's friend in the asylum, thinks she's a witch and "that she could heal with herbs, that she could cast spells both black and white" (4.18). Connie seems a little skeptical, and readers are perhaps inclined to be skeptical as well.
But in the book, Connie projects herself into the future and, for that matter, doctors have figured out a way to control people's brains. Are those things really more realistic than witchcraft? Is Sybil crazy because she believes she's a witch? Then what about Connie, who believes she travels to the future, or the doctors, who believe they've cured Skip?
The book is in part about the way that reality is defined by who has the power, and madness is defined by who the doctors are (or who has the right to commit you). So witchcraft in the novel is a little nudge for the reader. You believe in this, but not in that? You think she's crazy, but not her? It's a way to get you to question your assumptions—in the novel, and maybe elsewhere as well.