In I Am Malala, why is there a preface and a prologue?

The preface (“Birmingham, England, June 2015”) introduces the subject of the book and establishes the continued importance of Malala's work through the Malala Fund. In contrast, the prologue describes the events prior to the shooting and thus invites us to read Malala’s story. Basically, the preface establishes Malala's credibility in the area of girls' education, while the prologue provides the background details that led to the shooting. Note that the preface is never a part of the text; it is numbered in Roman numerals rather than the Arabic numerals used in the prologue and the rest of the text.
The preface provides details about Malala's adjustment to life in Birmingham and describes her work with the Malala Fund. From the preface, we learn that Malala played an instrumental role in building a school for the Maasai Mara people in Kenya. The preface also explains why she visited Nigerian schoolgirls on her seventeenth birthday: in 2014, more than 250 schoolgirls in Chibok were abducted by members of Boko Haram. Although many of the girls have been rescued, the Nigerian government is still working on rescuing the rest of the girls. Meanwhile, Malala aims to support girls' education in Nigeria through the Malala Fund. To date, Malala's foundation has funded education projects in Jordan, Kenya, Pakistan, and Nigeria; it has also worked to integrate Syrian refugees into Jordan's schools. So the preface helps us understand why an organization like the Malala Fund is necessary, while the prologue briefly explains the premise behind the shooting (the main event of the book).