George Orwell in his essay 'Why I Write' outlines a number of reasons why he (or one) writes.
I don't think he lists all the reasons to write. For instance, the spiritual impulse. (I guess he might that consider that subsumed under some of the other categories.) And the exploratory -- writing to discover. (He might also consider this as belonging to the other categories.)
What other reasons are there to write?
I mean beside supporting your agents' knitted moustache fetish and the inevitable expenses it entails.