The author is emphasizing the sense of loss that Della must feel when she realizes she can’t use them.
In the excerpt from "The Gift of the Magi," the author O. Henry directs attention to the beauty of the Della's combs that Jim buys for her. They are special, splendid and expensive, and she has always wanted them. In fact, she never thought that she would ever possess them. As a result, the author intends to highlight the irony of the story, which is that she will not be able to use the combs she has dreamed of.
Author's purpose description of the beauty of the combs is the author is emphasizing the sense of loss that della must feel when she realizes she can't use them