Image means a lot. People want to have a specific look and that look is usually dictated by the media. Since most of these models and celebrities are thin, women feel the need to imitate that look. So ye, thin models and celebrities do help with the sales of products to women.
Thin models most certainly help to sell apparel and accessories to women. Most women want to be as thin as possible, even though most women are also on the chubby side. Just as most men want muscles even when they don't have much, women will take someone more seriously if she's closer to their ideal.
Using thin models and celebrities to sell to women is what it's all about. It's a projected image of what the advertisers think women want to look like. They want them to get the idea that if they use that particular product, they will somehow "transform" in to those thin and beautiful models. Totally unrealistic and illogical, but it works.
Women and men alike now how ridiculously unrealistic models portraying how you're "supposed" to look are, while many women would like to look like them they know they're not going to and they don't have delusions of something being modeled by one looking the same on them. All thin models do is further damage our perception of what's attractive.