Belle never appears in person throughout the novel; she has married young and impulsively; her actions and fate are imporatant to a number of characters. For example, Woodrow "longs for her", Love feels badly that he was "not kinder to her" and Grandpa and Granny "wish they could have helped her."
Porter explains to Gypsy, "She actually vanished many years ago, when she was about your age. Now she is out there trying to find herself again."
The message, therefore, is instructive to characters who wish to avoid a similar fate of disappearance in their attempts to "find themselves."