American culture

What is the significance of primary sources in the construction of historical narratives? Considering the Zinn readings and the two films in particular (one of which would be classified as using oral history and the other a recapturing of marginalized historical figures' tales through primary sources), what do primary sources add to these stories? What do they do that is different or distinct from a secondary source like a history textbook? (For example, how would Unchained Memories differ if they were just telling you about slave life vs. doing so in the words of people who lived as slaves?)