One thing I've noticed about readers and writers, when it comes to Prologues--and Epilogues--they either like them or not. There doesn't seem to be any middle ground. If you read my novels, you know I like using both.
I view each as a perfectly legitimate literary device that, when done well, can enrich a story. Prologues serve different purposes for different books, but whenever and wherever used, they have specific jobs to do. I've employed them in a number of ways: to point to the story itself, providing a hint of the problem or suspense or conflict to come; to offer a glimpse of some significant facet of characterization that would be difficult to present later on; to acquaint the reader with an experience from the character's past that's pertinent to his actions in the story; to introduce an "icon"--an object or symbol that will continue throughout the novel or the series; to pose a question or a hint of intrigue that will lead the reader into the story; to set the tone or the atmosphere of the story.
Some examples: in Prelude, the first book of my American Anthem series, the lead character of the entire series, Michael Emmanuel, is introduced as a young boy aboard a ship in New York Harbor as he prepares to leave for his home in Italy. He has an experience in which he hears a glorious music that he longs to "catch and hold"--an impossible feat. This experience, this desperation to "catch the distant music," informs Michael's ambition, his aspirations, and even his faith walk throughout his youth and his adulthood. In the second book--Cadence--the prologue features a pivotal moment in Michael's career on stage by pointing to a drastic change that will affect not only Michael's life, but his wife's as well. And in the third book--Jubilee--the prologue depicts the birth of his daughter, while posing not only a question about the future but also offering a bittersweet promise. Each event occurs "outside" the time frame of the story, yet is intended to take the hand of the reader and tug her into the story.
In my Emerald Ballad series, the prologue of each book had to do with the "icon" of the series--the Kavanagh harp, which was passed down across the centuries from one generation to another. Each prologue progresses chronologically from century to century as the reader follows the role of the harp in the lives of the main characters, both in America and in Ireland.
In A Distant Music, the first book of my Mountain Song Legacy,the "stage is set" to introduce the icons of the series--a silver flute and a penny whistle--and their significance to the main characters ... and an entire town.
In The Riverhaven Years, the prologues take the reader into the heart and memories of the main character--Rachel Brenneman--to hint of a mystery and a problem that will haunt her throughout the progression of the series.
Perhaps you've noticed that many writers of mystery and suspense novels use the prologue to show the commission of a crime or hint at the suspense element of the novel. In women's fiction, a prologue might depict a past event, setting up its relevancy to the story to follow. Some romance novels might provide a scene in the past of two characters, perhaps their "last good-bye," then begin the first chapter with the story unfolding to show the return of one of those characters to his or her hometown and the first encounter or reunion with the former sweetheart.
Prologues are extremely flexible; they can occur in the past or in the present--or even in the future. They can introduce, set a stage, show an act of violence, hint at future events, provide brief backstory, pose a question that needs to be answered, establish the narrator, show an experience that influenced a character, or--one of my favorites--set the mood or tone of the story.
More than anything else, I consider a prologue important for the purpose of intriguing the reader, luring her into the story. You might call it a setup.
As for the role of an epilogue ... well, that's for next time.