Honest, open, point of view: passion & research...
Documentary filmmakers are almost always driven by a passion for the subject and come to conclusions and judgments about the subject at hand after an extensive research method and on-camera observation.
Viewer becomes engaged…
“When you see somebody on the screen in a documentary, you’re really engaged with a person going through real life experiences. So for that period of time, as you watch the film, you are, in effect, in the shoes of another individual. What a privilege to have that experience.” -- Albert Maysles, US documentary filmmaker
Digs deep into a topic…
We all have pretty entrenched views about certain things, but documentaries are really good at revealing new things about history, politics, society, culture, and other important topics. That’s because a good documentary filmmaker will take a subject that is already reasonably well known and dig deep to find often surprising facts and insights. To plumb why the story matters.

See also 20 Reasons Why Documentaries Are Important