The story is "The Last Leaf" any help is appreciated

What, in your view, is O. Henry doing by including this passage, describing what Johnsy was doing? How does this help his story? “Johnsy's eyes were open wide. She was looking out the window and counting — counting backward. "Twelve," she said, and little later "eleven"; and then "ten," and "nine"; and then "eight" and "seven", almost together.