What incident caused Jeannette to start an escape fund, and why is this important?

Rex gives her a whipping, so she starts an escape fund because she vows she'll never be whipped again, and that she will get herself out of Welch.

"When Jeannette talks back to her mother, telling her that she cannot quit her teaching job because they need the money, Rose Mary asks Rex to discipline her. Jeannette tells him that he too has been shirking his parental duties and Rex gives her a whipping with his belt. After this, Jeannette resolves never to receive a whipping again and she begins to save money for her escape out of Welch."

I believe Jeanette started an escape fund its because shes had enough of her mother and fathers filthy outrageous life style and wants to become something more and breake free from this psychological barrier they hold over her.