"Well, I was just wondering, anyway... I see that you are midway through a romance of some kind, a novel... Are you enjoying it?"
"Has it satisfied you?"
"Not yet, oh creepy stranger, not yet."
A book is not one pleasure, but many. Highlights include fine phrases; intriguing characters; hobbits; escape; excitement. All of these are enjoyable in themselves and for their own sakes, but they do not necessarily result in satisfaction. We only get that when one of our needs or desires have been met and a great many stories are designed to satisfy a craving that they themselves have created. Thus, we learn about the One Ring, we become fond of Frodo and suddenly, we have a burning yearning(TM) to get to the end of the quest and to find out what happens.
So far, so very, very obvious.
Equally obvious, of course, is the fact that any writer failing to bring the circle they have opened to a close, will have their book thrown into the fireplace and will face torches and pitchforks 'ere harvest.
But it's not enough to complete the plot, however. When you write the evil overlord's demise, you might think the story is over, but does your customer feel the same way? In order to close the deal, to avoid that most painful of words, "meh", it's not enough to satisfy the intellect -- the subconscious needs feeding too.
I am often intrigued by the way foreshadowing works to open and close circles that the reader isn't even aware of. For example, if you start your tale on a snowy day and end it in the same way, it will probably feel more complete. It's like you've put a border around your story so that your reader's subconscious knows exactly when she can let out her breath. It's over. It feels like it's over. There's no hangnail to gnaw away at her peace of mind.
Have you ever noticed how perfectly normal phrases feel so much more profound when somebody famous has written them before you? Look at all those books that use Shakespearean quotes or bible verses for their titles. They sound... grand, but that's only because of brand recognition. Many writers use foreshadowing to produce their own resonances.
Musicians use it too, of course. In an opera or a movie, particular notes or chords prepare our subconscious for the appearance of certain characters. It makes them feel more real, more three dimensional. If an oboe plays every time gollum comes on stage, it will be as if the poor creature is having a tangible effect on the world around him, as if he's real.
Foreshadowing is a necessary tool if you want your story to have impact. Anything that comes out of the blue will appear fake, but because foreshadowing works best when it affects the subconscious, you can't signal too obviously. At the end of the day, we're all just trying to keep our heads off a peasant's pitchfork.
"So, are you satisfied yet?"
"Not really. It was a bit of a ramble. Didn't seem to go anywhere."
"Can I borrow it, then?"
"No, I don't even know you, but already I hate you. I'm off."
"Wait! No, you can't...."