why does twain mention the slave girl roxana and her duties for percy driscoll?

One of Percy Driscoll's slaves. Roxy is charged with caring for her master's son, Tom, along with her own child, Chambers. Fearing that her child will one day grow up and be sold down the river, Roxy decides to switch the babies. Upon Percy Driscoll's death, he grants Roxy her freedom, and she decides to become a chambermaid on a steamboat. Later, she consents to her son selling her back into slavery to help pay off his gambling debts. Twain uses her as an example of generations of lives torn apart.