What is the point of deconstruction in fiction?

Every so often, you come across a book or television series that deconstructs a fairly common plot and/or setting, generally shifting the tone from whatever it was to something much darker.
I can see how one might conclude that deconstruction may be necessary in terms of the real world (I’ve used it myself), but using it in fiction – especially non-allegorical fiction – just feels like the author is trying to rain on everybody else’s parade – the entire point of fiction is that the parameters of the setting may be defined by the author. It is not reality in the first place.