Answer: Because it showed that there isn't a single standard for beauty - it broadened the definition of beauty.
Step-by-step explanation:
Dove embarked on a campaign of Real Beauty where they used women from real life instead of models to show that there are many kinds of beautiful.
Before Dove did what they did, it was standard practice to use models for such ads. These models would be so beautiful and yet still be brushed up to look even more so. This did not represent the realities of most women and could lead to women watching them lose confidence in their looks and self.
In using real women, Dove showed that you didn't have to be a model to be beautiful and this message was quite well received and led to them receiving high praise (and some criticism) from many quarters including from One Club.