There are several distinct answers to this question, but one type of answer in particular that I am interested in.
First, to go over some reasons to do art that are more straightforward:
One reason to do art is for profit. We might be in a position to make money to support ourselves through selling the art that we produce. In this case, then, art would have instrumental value for us -- it would have value in so much as it gets us other things which we value.
Additionally, we might do art as recreation, enjoying the process of creating a work of art. In this case, then, art would have subjective value -- we would value it because of the particulars of who we are. It could be argued, however, that this type of justification for doing art falls under the instrumental category, because it is valuable because it leads to happiness -- it is instrumental in bringing us happiness.
A third reason why we might do art is as therapy. Although this may be closely associated with doing art for pleasure, there is a difference in character do to the fact that many times art therapy is being prescribed to individuals by outsiders when these people are suffering from psychological problems. Doing art in this case is again instrumental, because it leads to something else, namely psychological health.
So here are at least three reasons to do art: for profit, for fun/happiness, and as therapy.
What other straightforward reasons to do art are there? (art as distraction?)
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Now we are to the point that interests me: why else do art?
We might do art because art is itself valuable. Just as many have proposed that happiness or preference satisfaction is valuable, we might consider art valuable. In this case, then, we really ought to do art, and art is thereby objectively important, independent of whether we believe it to be important or not. If we believe this, we can then say that if we are not doing art, we are often making a mistake.
We might, however, believe that art is not itself valuable. However, in addition to not believing art to be valuable, we may also think that nothing is valuable. This position, then would be that of an ethical nihilist -- one who believes that nothing has first order value. Yet such a person may admit, and I believe it would be rational to admit, that they cannot be certain of such a position. Therefore, by default, the most rational course of action would be to act according to their secondary beliefs concerning value. Such secondary beliefs may include a view that art does indeed have value in itself, and therefore, though the individual may not strictly believe that art is itself valuable, it would be rational in such cases for them to act as if it did.
Lastly, I want to consider the more poetic view that we might do art for no reason. Although in such a case, the individual may hold that there is value yet art does not have it in itself, it is more likely that someone considering such a position would believe again that nothing is valuable. This is because if we believe there to be value, yet we do not grant it to art, we would usually not concern ourselves with art, but rather those things that do indeed have value. What are we left with then? For those who believe there to be no first order value, art would be like a libation poured to the gods, without the gods.