Why do it? Why create anything?
I always loved Stephen King's answer.
[From the introduction to SKELETON CREW]
"You don't do it for money, or you're a monkey. You don't think of the bottom line, or you're a monkey. You don't think of it in terms of hourly wage, yearly wage, even lifetime wage, or you're a monkey. In the end you don't even do it for love, although it would be nice to think so. You do it because to not do it is suicide."
"The story paid me by letting me get back to sleep when I felt as if I couldn't. I paid the story back by getting it concrete, which it wanted to be. The rest is just side effects."