I ask my students to answer that question in the essays they pen for me at school. I ask every story I read to answer that question for me, rewarding me in some small way for the time and effort I put into reading it.
I'd like answers (who wouldn't?), but most often there are none.
It's one of the trickiest things I see in the writing I encounter. Many novels deliver a fine first act. The exposition is keen, the world building is engaging and the characters are authentic. But things often fall apart, and the second and third acts are thin shells in comparison.
I don't know why this is, but I can tell you that sustaining narrative continuity is hard for me when I'm working on a novel. The passion one has for a story in the early going can fade quickly, and it's definitely work to keep that fire going at the same level throughout the writing process.
Subsequently, many novels simply can't sustain the momentum and goodwill that they build in the first portion for the course of the entire book. I've blogged before about how much I admire Joe Hill as a stylist in the short form, but both Horns and Heart Shaped Box were disappointing because they ultimately failed to pay off the fine storytelling that was delivered early on. In the former, the plot unraveled and there was a rather clumsy insertion of a medical catastrophe that dumped the story on its head; in the latter, the chills Hill generated in those unsettling first pages never returned.
It just wasn't a very scary story after about the midway mark.
My students write many strong essays. They compose pieces that, for the most part, are eloquently phrased and nicely supported. Some of them really write well. But many (in fact, the great majority) can't write a conclusion effectively. They see the end of the essay drawing near and they abandon it, often writing trite, two-sentence summaries that fail to deliver anything resembling an actual lesson.
This is just the type of thing I'm trying to avoid in the novel I'm almost finished with (in terms of draft one--the editing never ends, it seems). I want the conclusion to have an impact. I want the ending to be memorable and to provide an answer.
And man, that can be difficult, because it's so easy sometimes just to say Hell, this is pretty good. I'm finished.
I'm trying to ignore that voice. It's hard, but I'm putting up a decent fight. Only the readers will be able to say with any confidence whether I did a good enough job of ignoring it and putting in the work...