What is the purpose of douglass describing the better treatment of the city slaves compared to the plantation slaves in the narrative of the life of frederick douglass?

 Frederick Douglass's narrative is a biography of his own life when he was a young slave living in Baltimore. He expresses that he felt fortunate to live in the city, instead of living in plantations, since by living in the city, he was treated better and it was an easier place to escape freedom. In the countryside, slaves were often brutally whipped and rarely given enough food or clothing. However, slave owners in the city would have been ashamed that their neighbors had seen their slaves without food or clothing. In the city, Douglass learns to read and meets a wide variety of people who help him on his path to freedom: white children who help him learn to read and write, sailors who teach him a trade, and people of the North, that make him see that not all whites own slaves.