Why would an author write an introduction to a nonfiction book?

to present the topic of the book to the reader
to thank people who contributed to writing the book
to list the resources used when writing the book
to provide page numbers for each of the book's sections

This would be done to present the topic of the book to the reader

Explanation:

The general reason for the use of a prologue or an introduction in any work of art is to present the topic of the book to the reader.

The theme of the book even if it is non fiction would be specific as it is in most of the books so it is better for the reader to have some idea of what they are going to get inside the book from the outset.

This only has to set their expectations in the broadest sense of it and not rally give away much about anything in the actual narrative