I'm all for writing the projects that we need to write, listening to the inner voice, and so on. I think my best projects have chosen me as much as I've chosen them.
But I like to think that books also contain something for the reader: something to uplift or amuse, encourage or entertain. Something to open their eyes to new experiences, or make them feel less alone as they recognize their own experiences on the page. Something to make them feel that they haven't wasted their time; they've come away richer for the time they spent with our words.
I suppose I'm talking about theme. I don't start writing with theme in mind, but I generally find the theme at some point during drafting or revision--otherwise, I sense that something's missing. Theme is the thread of coherence running through the story; theme is what makes the story make sense, gives it its significance. So often in life we don't know why things happen the way they do; we don't get the point. But with stories, it's very satisfying when we can identify the point.