The fact that the book is completely truth is imperative to the validity that the narrative holds. If this slave account was changed or modified, readers could rebut with the idea that the author was only exaggerating the events and emotions to appeal the reader. You can dramatize something for an effect, but because it is all true, the brutality is real and creates an image and puts a name on the experience for the reader. The whole true is the only text that holds credibility.

Also the "unrefined" language and style contributes to the merit. When actual dialect is used it again creates an image that can put the reader there and give them the closest account of slavery possible for someone who is living well beyond those days.