Does the story of Roseto establish Gladwell's purpose in Outliers?

Yes. Gladwell sets the stage at the beginning of Outliers by first defining what an outlier is, and then by following it up with the story of a small town in eastern Pennsylvania named Roseto. He says an outlier can consist of “a statistical observation that is markedly different in value from the others of the sample.” This description fits the residents of Roseto, who had fewer instances of heart disease (and of other diseases) than people living in other parts of the country. A team of researchers went to Roseto to see how these individuals could be so much healthier than the rest of us. They studied their genetic make-ups, diets, and lifestyles. Most of the people had emigrated from the town of Roseto Valfortore in the Italian province of Foggia. It turned out that they had brought their native paesani culture with them. This tie gave them a stronger sense of community than other American towns had. The people spent a lot of time together. Generations lived under the same roof. People worshiped at the same church and belonged to some of the same charitable organizations. Their good mental health and personal satisfaction was reflected in their good physical health. Gladwell explains that the town of Roseto was an example of an outlier: “a place that lay outside everyday experience, where the normal rules did not apply.”