The author wanted to introduce the book’s background and his purpose for writing.
Explanation:
N. Scott Momaday's memoir "The Way to Rainy Mountain" is a form of travelogue as well as a historical documentation of his attempt to trace his heritage. It also serves as a way of reconnecting to his past, his heritage and his background.
In the prologue, Momaday begins with the description of the mountain and how it had actually been called that. The prologue also gave an introduction to why he was writing this piece, and why he needed to focus on that. Attempting to preserve the traditions and customs of his Kiowa people, Momaday relies on the prologue to give that purpose, focusing on the importance and greatness of the Kiowa people.