I know it is called a documentary, but you could also clearly call it just a talk show. That does not take anything away from the impact this might have. There are other shows that really put people on the spot and while doing a good or great thing (reuniting families or other nice stuff), work to get audience all sentimental and profit from that.
Oprah at least in this instance really cares for the 4 individuals who really had to go through quite a lot. It is amazing and heartbreaking hearing the people involved (at least the innocent ones, because the ones who pushed the negative agenda, will not speak out - even if they did, what would they say? Sorry? Would be a start I reckon, but don't hold your breath ...) and their journey and what they thought about the movie.
You also get to hear from the actors and how they prepared for their roles and what it means to them. Obviously it makes sense to watch the movie/series beforehand. Netflix actually made me watch this after the the 4 parter was over. Usually it asks you what you want to do next ... in this instance it just started after the other one ended. I'm not complaining mind you, because it is quite the complimentary piece ...