I have had little to no interest in a movie that claims it is "Based On a True Story" since Fargo. It's well known that it was a lie that Fargo was based on a true story. There are many movies though that alter so much of the real story though that they end up more fiction than reality. So what is the point of putting "Based on a True Story" in the movie description?
Lots of reasons:
In literature, sometimes an author (think Erik Larson) will publish a book as non-fiction even though it has fictionalized dialogue and a certain degree of supposition, which I find equally annoying. James Frey's memoir A Million Little Pieces contained enough fabrication that libraries don't know where to put it! If the story (movie or book) is interesting enough, "based on a true story" always makes me want to research the true story.