That is a very good question. Imagine that you were reading a history of World War II as written by Adolf Hitler and that it is the only text available on the subject. What do you think your opinion of the war might be when you've finished? We need to be aware of all perspectives in order to find the truth of the matter. All sides contain a portion of the truth, but they also throw in their own perspectives, their own prejudices, their own propaganda.
The eNotes critical overview of the book Kindred notes that the author not only emphasizes "the power of those who are oppressed, it also reclaims history from the dominant culture." If all we knew about slavery in the United States came from white writers, we would not know that those slaves were real human beings. We would not know the many influences those African Americans had on society. We would not know ourselves.