 What is the main purpose of a “galley proof”?
Virtually all ambitious trade hardcover books are preceded by an edition of 100 to 200 flimsy "galley proofs." There is an ethos of letting the chips fall where they may about the sending out of galleys: Now the book will begin to receive uncontrolled reaction. The effect was, first, to increase the allure of the book (There must be something really hot in there!), and second, to ensure that no one inclined to be skeptical would be able to weigh in at the moment of publication. The people who had galley proofs were handpicked by Murray and his publisher. Most works containing fresh regression analysis and historical argument from primary sources would be published in academic quarterlies that send manuscripts out for elaborate, lengthy evaluation before deciding whether to publish them.
 To generate buzz about a book before its publication.
 To give editors a final version to proofread.  To give readers a chance to pre-order the book.  To offer experts an opportunity to critique the book.  To generate buzz about a book before its publication.