As a marketing tool, publishers provide free copies of new titles to booksellers, journalists and even celebrities.
Such books are variously referred to as readers editions, an advance copy, an advance reading copy, ARC or ARE. It's the book privately released by its publisher before the book is printed for mass distribution.

Readers editions generally lack the final dust jacket, formatting or binding of the finished product; the text of an advance edition may also differ slightly from the market book, after comments are received from the reading group, or late errors are found in the manuscript. When a celebrity reader or journalist gives an endorsement, that's added to the back cover and other promotional items.
Reader books are normally distributed three and six months before the book is officially released to reviewers, bookstores, magazines, and (in some cases) libraries.
Most national magazines and major reviewers work on long lead times and arrange future issues months ahead of publication. For example, if you've written a book on healthy living you may want to consider a January publication date, when many magazines focus on health and weight loss. The galley would need to be sent no later than September or October when editors are planning the features that will appear in their January publication. If you present a book on fitness and weight loss to an editor in mid-January, it may simply be too late as they're already planning their Mother's Day issue.
Newspaper, television and radio outlets generally prefer to run their reviews and coverage as close to the actual publication of a book as possible. It's also important to have an advance copy of your book in their hands before the final copy is available to allow them planning time.