I would argue that the purpose of Educated: A Memoir is to highlight the importance of education and rationality. Having been raised in an isolationist Mormon compound in Idaho, it would have been unlikely that Tara would ever have gotten an education, but as she studies at Brigham Young University and later pursues her doctorate at Cambridge, her eyes are opened to new ways of thinking.
As a child, Tara's father, Gene, forbids her to go to school or even have any modern medicine. She is home schooled, but this takes a backseat to daily labor. The family focuses on self-sufficiency, as Gene believes that economic collapse is imminent.
It is thanks to her older brother Tyler, who learns from any texts he can find and has a passion for music, that Tara is introduced to the idea of life outside the compound, and to the idea of getting an education. She becomes determined to follow in Tyler's footsteps and gain an education, and it becomes her sole focus, even as she carries out the tasks that are expected of her.
When her father's extremism becomes more prevalent, Tara begins to seek work outside of working for her father, which drives home the theme of independent thinking and self-sufficiency. This brave step towards independence is an important part of the sequence of events that led Tara to become an independent woman who was determined to pursue her education.
Her controlling father and her abusive older brother, Shawn, are juxtaposed against Tyler, the gentle brother who encourages her to get an education.
It would be easy to argue that the purpose of this memoir is to cast Mormonism in a disparaging light. By the title alone, however, one can tell that the purpose of this great memoir is the power of education.