A bildungsroman chronicles a young person's journey into adulthood and shows what molds him (or her) into the person he becomes. A bildungsroman will often use the convention of starting with childhood and proceeding in a chronological way through the significant events of a young person's life. Dickens' David Copperfield and Great Expectations are examples of that format—and they, too, like Douglass' narrative, are told in first person.
The main idea Douglass wants to convey is that, no matter what Southern whites might say, slavery is a terrible and dehumanizing institution that should be abolished—primarily for the benefit of black slaves but also for the benefit of whites, whose souls are corrupted and deformed by the excessive power they have over other people's lives.
Douglass uses the narrative strategy of arguing against slavery through the example of his life story. A picture of an individual life of suffering raises our emotions more than any number of facts and statistics. Douglass' novelistic description of his aunt's brutal beating by her master, the blood running down her back, and the traumatic effect it had on him, as well as his own lack of shoes in the coldest of winter, his struggles to learn to read in Baltimore against all the odds, and his ever-firmer conviction that it was wrong for him to be enslaved puts readers on his side and convinces us that slavery is a great evil.
Douglass also weaves into that narrative compelling objections to a number of arguments against freeing slaves, such as they say they are happy in slavery, that they are well cared for, that Christianity makes owners compassionate, and that the slaves seem carefree when they sing. Douglass' technique lies in the details he shows us and the passion he conveys, engaging us as a storyteller while demolishing the arguments of his opponents through the force of anecdotes. His message is palatable to what would have been his white audience because it is tied to the struggles and yearnings of an innocent young boy with the same mind, heart, and spirit that a white boy might have.