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 wealth and privilege, and the girl, who is in a vulnerable position. The metaphor of "capturing" suggests the imbalance of power in their relationship and how Rosemary's actions, though well-intentioned, still stem from her position of advantage.