Writers who write "for art" write to please their own aesthetic sensibilities. They focus on their own tastes, not the tastes and expectations of an audience.
Those who write "for commerce" write to please others, and a focus on the needs and expectations of a particular audience is strong.
For example, the novelist Graham Greene divided his fiction into two categories: his serious work, which he called novels, and his popular work, which he called entertainments. Similarly a poet might write verse for Readers Digest, then turn around and write serious poetry with a more limited market in poetry journals. Dorothy Parker refused to call her verse "poetry" because she believed it didn't reach that high standard. Emily Dickinson, of course, is the prime example of the poet as artist, putting her poems into a drawer, writing primarily for herself.
I bring this up because here is another area in which screenwriting differs from other forms of writing: there is virtually no "writing for art" tradition in screenwriting. In this sense, screenwriting is totally directed toward commerce, toward relating to an audience. There is a small exception to this generality: art film filmmakers who write their own scripts. But for the screenwriter who is not also a filmmaker, there is no tradition of writing screenplays "for art's sake." Screenwriting is highly collaborative, and the collaboration is aimed at putting films in the commercial marketplace.
11/02