Answer: It helps convey the idea that the woman is constantly faced by her responsibility to care for others.
"Daystar" is a poem by African American poet Rita Dove. The poem illlustrates the life of a woman who is constantly busy taking care of others. She feels trapped in her roles of mother and wife, and the only relief she can find is an hour a day while her children are napping.
The image of diapers steaming on the line shows that, even when the woman wants some space to think and be herself, she is always reminded of the responsibilities she has towards others.