A long and thoughtful article on The Reasons for Poetry by William Meredith is available at http://www.poets.org/viewmedia.php/prmMID/16176
I quote a section here:
If every poem is new, it is also associated in its own mind, and ideally in the reader's, with other poems of its species. Poems hold one another in place in our minds, Robert Frost said, the way the stars hold one another in place in the firmament.
The three roles I envision are these:
1. The poet as dissident. Underlying poems conceived by the poet as dissident is a social criticism, whether of a tyranny, like George III's or Stalin's, of an abuse, like nuclear pollution, or of a system, like capitalism. As an activist poet, the dissident is likely to be formally radical, since the large metaphor of his work is revolution, but not necessarily.
2. The poet as apologist. Underlying poems conceived by the poet as apologist is acceptance or approval of the human and social predicament of his tribe. However much the poem may focus on errors or imperfections in its subject, there is implied an order or decorum in the model. Often the poem's mode is praise, overt or implicit, of the specific subject or of the human condition. Every work of art, the Christian apologist W. H. Auden said, is by its formal nature a gesture of astonishment at that greatest of miracles, the principle of order in the universe. The poet as apologist is apt to have a pronounced sense of form, but not necessarily.
3. The third and commonest stance of the poet is the poet as solitary. While the poem by the poet as solitary will sometimes take the stance of talking to itself, more often it speaks from the poet as individual, to the reader as another individual, and intends to establish a limited, intense agreement of feeling. There is no implicit agreement about social needs or predicaments. Such solitary experiences, and they make up most of lyric poetry, carry on their backs the world they are concerned with, like itinerant puppet-shows They create a momentary event where the poet and the reader dwell together in some mutual astonishment of words. The best teacher I ever had told us a lyric poem can only say one of three things. It can say, "Oh, the beauty of it" or "Oh, the pity of it," or it can say, "Oh."
This is a crude trinity, and if useful at all, useful at the elementary level of detecting and dispelling false expectation. I will rehearse the three roles with some examples. [He continues ...]