The most likely reason the author uses the figurative language “she gazed at the little captive she had netted” is:
The language reveals the power that the rich had over the poor.
This interpretation highlights the dynamic between Rosemary, who represents the wealthy, and the girl, who is in a vulnerable position and dependent on Rosemary's kindness. The term "captive" suggests a power imbalance where the rich have control or influence over the poor.