I think that the term "Art" comprises a great field but always means expression of feelings/moods. No matter what kind of art this is. Literature, photography, drawing - dancing and singing - everything is expression of the feelings. In your feelings you may have (untransformed) dreams - those might become your fantasy works. If you write about your feelings and experiences is more a biography or reality work. And so you can find more and more traces that connect your feelings to art.
So art for the art. Express your feelings to express them, to show them, share them with others
When someone only expresses feelings for commercial benefits, passion will get lost more and more. The more passion you have to live your art, the better you are doing your job. Following something without passion is living without joy. You will be only half as good. So do never sacrifice your expression of art for profit reasons.
Concerning therapy. There is a link to art as well. Most people do not know how to express their feelings. Art will help them. Others again cannot talk about what they have experienced but can draw it e.g. by expressing their feelings in art but not directly to a person or the public. Drawing and/or writing helps to process unprocessed events and experiences and makes it easier to get over them. A confrontation with unknows fears or horrible events via the means of art. Curing people can also belong to art's sake.
Art's sake can only mean something positive, hopeful, limitless, healing, beauty, creativity, passion - but also sadness and anger. It is better to write than to break things or to kill people. What do you think?
Positive side-effects of art is surely that some people are impressed by someones art or envy or admire or get hope or simply hearts and eyes are open to listen - so many things could be art. It should not be reduced to one single purpose
(I'm guessing this is going to come up a lot, so I'm arguing against it now, feel free to disagree, obviously.)
The ideal, for me, is to do art for the sake of art: push something to the point of perfection because I can and want to. I don't see why it's any more pure or noble than any other motive, though. It just happens to be the one that meshes best with I DO WHAT I WANT.
I started this thread because of all the quagmire surrounding people who apparently see being a successful professional writer as a "dirty" thing. I think it's part and parcel of this culture of valuing "art for art's sake". And it's so odd because I can't imagine how we all got here after the Dadaists. Maybe it's backlash.