In the novel, A Wrinkle in Time, the three women, Mrs. Which, Mrs. Whatsit, and Mrs. Who, are all stars who, after fighting the darkness in space, have come to Earth to help Meg, Calvin, and Charles Wallace defeat the darkness and save Meg and Charles Wallace's father. They have disguised themselves as elderly human women, but because they are really stars they have a hard time fitting in and getting their disguises right. Mrs. Whatsit is the youngest and therefore has the easiest time adapting, but Mrs. Who and Mrs. Which struggle a bit more. Mrs. Who, in particular, struggles to make herself understood and so relies on quotations to get her meaning across. She also does it in part to help keep everyone's spirits up.
We find out in the novel that Mrs. Who, Mrs. Whatsit, and Mrs. Which are stars who have come to earth in human form. They pose as old ladies, and part of the humor in the novel is that they don't quite get their disguises exactly right, coming off as odd and eccentric. For instance, their names are not quite normal: how many people have as surnames Who or Which?
Mrs. Whatsit explains why Mrs. Who speaks in quotes after Charles Wallace says to Mrs. Who in annoyance that he wishes she would stop doing so. Mrs. Whatsit says to him:
But she finds it so difficult to verbalize, Charles dear. It helps her if she can quote instead of working out words of her own.
Mrs. Who then chimes in that she does it, in part, to be funny, saying that we all need to keep our sense of humor in serious times.