Sometimes it's very easy to see how we got to the beginning of a story. It couldn't get much more clear, and any lengthy explanation would be boring.
Other times, however, a little information is a good thing--an igniting past or present event--an exposé on something happening concurrent with our story. If this little bit of information mightily feeds that story, it has validity.
If it's just a bunch of backstory or an information dump it's worthless and as a reader, I'm ready to walk.
I'm amazed at the number of people who skip prologues. Puh-lease. Are you kidding me? I have a passion for prologues. IF THEY SERVE THE STORY.
As a writer, I have a prologue in the current ms I'm working on. It's not backstory. It's a murder. Would you skip a murder scene? Sheeshkabobalino.
But here's the real truth. If you write a prologue, it better be important. I mean, MAJORLY important. Not just "good information." That prologue better pull the reader in and give them satisfaction--both initially and how it relates to the story. And Chapter One? That first sentence better do the same thing. Don't be a wuss. NEVER think that you've done your job by hauling someone in to the story with an amazing prologue, and then spending the next gazillion words writing a different book.
I'm convinced that some readers skip prologues because they've read so many unnecessary ones. Don't add yours to that pile. Make it vital--or take it out.
It's all better with friends.