But, what is the importance of the authors note, and is one always needed?
They are not always needed.
Such notes may explain a bit about the book's premise, inspiration, or maybe the writer just wants to wave hello and say thank you.
They're not always important. I find the longer they go on the less important they are. Unless it's a REALLY good writer anything longer than a half page is positively trivial. I've seen them run on to 20 pages, skipped them, and enjoyed the book just fine.
Sometimes I've asked for an Author's Note in front of a story to explain its context to a new reader in 100 words or less.
It would be something like this:
The Adventure of the Flying Pig is a Holmes and Watson novel set in A.C. Doyle's universe. This book was discovered under a brick in the fireplace of a bed and breakfast by the char lady, who promptly took it to Ebay and made a fortune. The author is dead, but were he alive he'd likely be very annoyed about the whole business.
Too often these days prologues contain the info dump a writer couldn't figure out how to incorporate into the main body of the story.
Unless (again) it's by a REALLY good writer who knows what she's doing, most prologues can be skipped or deleted altogether. That's my
opinion, I'm fond of it, so don't bother convincing me otherwise, and I will return the favor to the pro-prologuers.
I will accept an Author's Note over a prologue any time!