There are two reasons to write. Both greatly influence the qualities, as they determine the goals, of the products.
The first is personal gain — I wanted to limit this to money, but have to admit authorship requires enough pride to devote yourself to a task for an extended period. Mostly I have wanted to be an author (read: to have authored/to enjoy the fruits of writing), and therefore an expert. Within this range, of course, is a determination to achieve social status/fame, or to improve cash-flow.
The aim then is to give the audience enough of what it demands to receive whichever combination of status, power, and financial securities is desired.
The second I’m inclined to offer as ‘nausea’. Something needs to be said, and it can no longer be kept down. Whatever the results precisely, here the author can hold back no longer.