In the seris of books im writing 'The Tales of Bogsworth' my prologues are usually set before the story line and they give a breif but important glimse into past events which shaped and continue to shape the world. These are important later on in the story.
In reality you can get through my books pretty easy without reading the Prologues as the infomation they contain is later explored in greater depth during the books. But personly i think if you buy a book but dont read the whole thing whats the point in buying it.
Thanks