What book publishers do
The Book Publishing and Marketing is a complex process. With a big publisher you get editing services and art work suggestions. They you get galilees, which is a printing draft to look over. They starting printing your book and begin a marketing campaign. They send out press releases and try to get your book included in purchased from book store chains all over. They may even arrange for a book signing to prompt buyers to buy. This is a very simple outline of the process. They may give the writer an advance against potential royalties if they feel confident the book will sell well.