In "Daystar" by Rita Dove, the image of "diapers steaming on the line" because it helps convey the idea that the woman is constantly faced by her responsibility to care for others.
Within that same stanza from the poem, it states that "she wanted a little room for thinking," but then continues to say that she saw all the "diapers steaming on the line." So, even though she wanted time to herself to think, she knew that she had to do the chores and care for the children. She constantly had to be responsible for the well-being of her family.