The movie should be portrayed as an event that has actually happened or could happen.
Realistic movies have a lot more impact on the audience then movies that are obviously non-fiction.
Making the movie accurate to a real event gives the movie credibility. It isn't unlikely that some audience could have experienced what the characters in the movie is going through. This is especially true with war movies like Platoon, Full Metal Jacket, or Born on The Fourth of July. The end result of a movie that is realistic is that it is more likely that an action will be taken to solve the societal problem.