Essentially, preorders demonstrate to publishers, as well as booksellers and online retailers that there is interest and demand for the book. This can lead to a number of things, including greater promotional efforts by the publisher, more bookshelf display space, broader coverage in the number of bookstores stocking the book, and a boost in the sales algorithms which can result in expanded marketing. Preorders are also counted towards the first week of sales, so they can help boost a book onto a bestseller list.
And you can preorder at ANY bookshop/bookseller, not just the big online sites. You can preorder at your local independent bookstore simply be requesting that they do so. Not only is this supportive of your local bookstore, but it also potentially gives the author a boost because chances are the store will order more than just your single copy, because if you’re interested in the book then someone else might be also. Nothing makes authors more grateful than to hear that our readers are spreading their love and interest in our books to others.
The same goes for libraries. Ask your favorite local library to order a copy, or put you on the wait list to borrow it first. Authors love libraries, and their preorders help, too.
Normally, you don’t have to pay for your preordered book until it is shipped to or picked up by you. So if you were planning to buy the book anyway, preorders give everyone a boost—from author to bookseller—and make sure you get your copy as soon as it’s available. It’s a winning situation all around. And as Helen states, we authors definitely hold a special place in our hearts for readers who preorder. On behalf of us all, thank you!