The thought process leading to action is something like:
What are the facts? => How do I feel about them? => What am I going to do about it?
But it turns out these thorny political issues have been unsolved problems for so long because there's a lot of complexity and nuance to them. Action gets tied up and dies in the first stage, and you never get to the part where emotion and passion come in.
Fiction shifts this process to:
Assume these are the facts. How do I feel about them? => Is this a reasonably good model of what's actually going on in the real world? => What am I going to do about it?
I may not be qualified to discuss the structure of the Antebellum South, but I am qualified to discuss Uncle Tom's Cabin. And furthermore, I'm more or less as qualified as anyone else who's read it. Knowing more about the realities of the time would definitely deepen my understanding, but it's not a requirement for talking about the book from within the closed environment of the book. (Okay, well that's an approximation. Obviously someone with perfect recall for details wouldn't have a great conversation with someone who skimmed it, but the point is that anything's true if it's written in the book, even if it disagrees with real life.)
Through fiction I've been freed up to think about things in a second order way, and consider what I would do IF these circumstances are true. And I'd say that once you start actually imaging yourself being a force for change, it gets you excited enough to start looking for an excuse to do it.
The threshold of action has been lowered from "know everything," to "be reasonably sure."