Many movies have what I would call a Wrap Around Story, or STory Enclosure, that amounts to the excuse for the rest of the story. The Fifth Element would be one example; big bad amorphous evil coming to destroy Earth, and that is the excuse for the adventures. Great movie, but the wrap around is rather weak, undeveloped, and cannot stand on its own. Now take Lord of the Rings; simple wrap around story, destroy the Ring before Sauron destroys the world as he once tried to do before.

I saw a movie once (City of Ember, I believe) about another one of those “last city of Man on Earth” stories. No details on the whys and wherefores, and it was obvious that the author or scriptwriter had no idea what ti might be either. How did 6 billion people get reduced to a thousand, why bury the city instead of putting it at the top of some mountain, why have such an elaborate means of leaving the city (and why after the river rapids so obviously flowed DOWN towards the way out did they suddenly find themselves ABOVE the city on the outside, but that’s another issue). Because there was obviously no planning of what had happened to Humanity to bring them to this state, elements of the main plot became inconsistent.

The point is, the story enclosure has to be fully developed and detailed enough to stand on its own. It is the framework upon which your main story is hung, so if it cannot stand then elements of the plot which it supports may begin to unravel, holes in the story appear. You don’t have to reveal the whole backstory of the enclosure in the novel, but the author should at least know himself what it is in full detail; by knowing the reasons behind why things are happening, the story will have more cohesion and the reader will sense a better unity within the story (and maybe even begin to guess the full backstory himself).

It is far better to have too much background details on your world than not enough. Everything must have a reason, a logic, a history. No hand waving or generalities. You will even find that this extensive note-making will inspire interesting additions to your plot that you hadn’t planned on; just remember to map things out in a series of logical consequences based on your secret backstories.

Your readers will love you for it.