Symbolism, Imagery, Allegory
Before Scarlett goes off to the barbecue, she has to get her corset laced up. It's a painful process, requiring Mammy to help. Check it out:
"Hole onter sumpin' an' suck in yo' breaf," [Mammy] commanded.
Scarlett obeyed, bracing herself and catching firm hold of one of the bedposts. Mammy pulled and jerked vigorously and, as the tiny circumference of whalebone-girdled waist grew smaller, a proud, fond look came into her eyes. (5.33-34)
Scarlett is quite uncomfortable in the outfit—she says, "'Goodness but my stays are tight!'" (5.38)—but she, like Mammy, is also proud of her ability to fit herself into the constricted clothing. But symbolically, this is about way more than just a tiny waistline.
Femininity, and the place of women in Southern society, is, like the corset, very restricting. Scarlett chafes inside it, and yet at the same time, she is proud when she manages to strap herself into it. She doesn't like the way women have to be "helpless, clinging doe-eyed creatures" (5.55) strapped so tight into their enforced artificial femininity that they can barely move—but she also wonders whether there's "something in it" (5.55). This rigid femininity, after all, is culturally revered.
The corset, then, is both a burden and an aspiration. Being a Southern belle is really uncomfortable, and Scarlett can tell it's uncomfortable and doesn't like it—but neither she nor the novel is ever quite willing to just toss the corset aside and declare it unnecessary and oppressive. She may break the feminine mold in other ways (hey there, business savvy), but she also works within it, intrigued by what it can help her attain, particularly when it comes to using men to shore up her own future.