As far as I know there is no simple answer to this question. Some of us write for personal therapy. Some to mentally escape our “real” surroundings. Some of us run with the idea we are or could be the next multi-millionaire author. Some of us want to say “I’m a writer” at parties and then tell all how wonderful our next book will be.
Some of us write to better the world, some to explain a complicated process.
No matter why we write, we all share a common bond, the struggle to complete because we all know to be a writer is difficult and often lonely, but rewarding in the end – which is really why we write, because we must.