A reader of any text can only assume the reason behind an author's creation of a text. Unless an author provides a reasoning for the reader, all suggestions are assumptive. One can suggest why they believe why an author wrote a piece by justifying their answer by using textual evidence.
In regards to Margaret Atwood's poem This is a Photograph of Me, I would suggest that she wrote the poem as a way to get readers to look more deeply at the things around us. Too many times we as readers, and sometimes observers, tend to focus upon the things which are right in front of us. Instead of focusing upon the foreground, Atwood wishes readers to move their focus to the background. Readers think, because of the title, that Atwood is describing a true photograph of her. Instead, no person is actually depicted in the photograph.
Other critics have identified the poem as one which speaks to oppression of women within a patriarchal society. A link for this interpretation (and therefore stand on why the poem was written) can be found by going to the third link provided below.