This bit from Erica Jong's preface sums up the "why" of writing quite nicely:
If you are born to do this--poor you. And if you hope to get rich by it, double poor you. But if you understand that writing is about the possibility of altering, by one iota, the brains of people as yet unborn, then go for it. You may wind up poor, jailed, or even assassinated for your ideas, but you will have spent your life in a gallant pursuit.
I also liked this, from Philip Martin's Afterword:
In the end, writing skills are mostly absorbed, not learned. Like learning to speak as a native speaker, learning to write well is not just learning a set of rules or techniques. It's a huge, messy body of deep knowledge, inspired by bits of readings, conversations, incidents; it's affected by how you were taught and where you live and who you want to become. For every convention, there is another way that may work better. For every rule, there are mavericks who succeed by flaunting it. There's no right or wrong way to write, no ten easy steps. There are no initiation rites to take you into the dark hut and reveal the magic chants and secret knowledge. It's not secret; quite the opposite. It's mostly confusing if you try to listen to all of it.
The way to learn is the same as to master a language, to understand an eco-system. Immerse yourself, stick around and be part of it, listen to others with experience, look for a place to fit in, find your niche. And learn to live with messy contradictions. Writing isn't logical, it's a part of life.
That's pretty much what I was trying to impress on myself and the rest of the writing world, especially the unpublished, when I named this blog: No rules. Just write.
Here's the Amazon link for The New Writer's Handbook 2007.