Why does the author bring up the experience of Henrietta and Mary?

In the story of Henrietta and Mary, Douglass is pointing out the fact that although the slaves in Baltimore were treated far better than those in the country, there were slave owners in the city who treated their slave horrifically in the city. Henrietta and Mary were examples of this treatment, treatment that Douglass says is some of the worst he's ever seen.

There are, however, some painful exceptions to this rule. Directly opposite to us, on Philpot Street, lived Mr. Thomas Hamilton. He owned two slaves. Their names were Henrietta and Mary. Henrietta was about twenty‐two years of age, Mary was about fourteen; and of all the mangled and emaciated creatures I ever looked upon, these two were the
most so. His heart must be harder than stone, that could look upon these unmoved. The head, neck, and shoulders of Mary were literally cut to pieces. I have frequently felt her head, and found it nearly covered with festering sores, caused by the lash of her cruel mistress. I do not know that her master ever whipped her, but I have been an eye‐witness to the cruelty of Mrs. Hamilton. I used to be in Mr. Hamiltonʹs house nearly every day. Mrs. Hamilton used to sit in a large chair in the middle of the room, with a heavy cowskin always by her side, and scarce an hour passed during the day but was marked by the blood of one of these slaves. The girls seldom passed her without her saying, ʺMove faster, you black gip!ʺ at the same time giving them a blow with the cowskin over the head or shoulders, often drawing the blood. She would then say, ʺTake that, you black gip!ʺ continuing, ʺIf you donʹt move faster, Iʹll move you!ʺ Added to the cruel lashings to which these slaves were subjected, they were kept nearly half‐starved. They seldom knew what it was to eat a full meal.