The curiosity - Peeping Tom - value. Mental illness is weird; it's often taboo in discussions and so when we have the opportunity to experience it at a "safe distance", ie from the comfort of a cinema seat, it allows us to find out more without actually having to come into face to face contact with the sufferers. I guess it's similar to those "Embarrassing bodies" TV shows in that respect. We wouldn't really want to be in close quarters with three legged people but we're fascinated from afar....
It's pure sensationalism, over-dramatisation to make big bucks and especially when the films are based on true stories (and are then romanticised, as with ABM) This article by Everyday Feminism discusses how the media, including the Film industry, contributes to the stigma of mental illness. Sadly, by paying to watch these films, we're just prolonging the myths