Ostensibly, if you believe the subtitle, the purpose of the book is to pay tribute to his mother, Ruth McBride. The story is centered around the way that his mother worked tirelessly to raise a huge family in very difficult circumstances. The story also uses episodes where Ruth recounts her own life and her own family, making the story even more poignant and powerful.
I would suggest that his purpose is also to simply tell what is a very compelling story as most memoirs are set up to do. McBride emphasizes the importance and use of education in all of his family members' lives, so one could argue that it is another purpose.