The character of Belle Watling serves an important function in the story. For one thing, she epitomizes the enormous social upheaval caused by the Civil War, so much so that the distinction between respectable and not-so-respectable women has become blurred. Scarlett O'Hara deeply resents Belle, not least because she sees many similarities between them in terms of character and temperament. Scarlett wants to feel superior to Belle, but has the honesty to admit that she has no right to. After all, her behavior towards Rhett is hardly what one would consider appropriate for a fine, upstanding Southern lady.
Belle's character acts as a kind of moral lesson for Scarlett, reminding her that she has a position in society, a reputation to protect, irrespective of the turmoil of conflict. Scarlett is a very willful, headstrong woman, but at the same time, she needs to be aware of the dangers of straying too far from the path of social convention, for that way lies potential ruin. To this end, Belle Watling provides a useful, salutary warning.