Every account of history is from a human perspective. Even history textbooks, which are trying to be objective, show us what the writers value by what they include, what they leave out, and how they frame their historical accounts.
The particular benefit of first-person accounts of history—and the importance of fictional thought experiments like the novel Kindred—is in the subjective human perspective. In primary sources like letters, diaries, and oral histories, there is no pretending that one is capturing the whole story. It is a very specific story. But it is through the combining of many specific stories that we are able to see a clearer whole.
The importance of Kindred as historical fiction is in the way Dana is able to immerse herself in American history (and her history), and the reader's vicarious journey to the past through Dana. Not only does Dana consider the slavery of the antebellum South in the context of her present in the 1970s, we as readers consider both levels of the past in the context of our present. This is important because it shows us the ways society has changed and not changed, and gives us a context for the society we live in now. The racism of our present moment, and the 1970s racism that made Dana's and Kevin's families disapprove of their interracial marriage, are direct descendants of American slavery. Just as Dana can trace her personal roots back to Rufus and Alice's nonconsensual union, we can trace the roots of our 2018 America back to the massive impact of the institution of slavery.
To travel back in time as Dana did, and to form close relationships with both slave-owners and slaves, has the effect of humanizing the people of the past. Unlike the "objective" history books, which paint the people of the past as monoliths and therefore distance us further from them, first-person accounts remind us that people created, supported, allowed, resisted, and fought the institution of slavery. Kindred, as a fictional version of first-person accounts, not only gives us a clearer picture of the pain and humiliation that slaves faced, it also does the difficult work of showing slave-owners as complex and contradictory humans. Often history can dehumanize the "villains" in an attempt to show how far we've come, but it is incredibly important that we make the connection between ourselves and the people who caused or enabled suffering. Humans (flawed, complicated, and afraid as we are) are entirely capable of monstrosities like slavery, especially when it is the status quo, and the majority goes along with it. Studying history, and understanding what we are capable of, is an integral part of changing society for the better, now and in the future.