On the use of detail in stories:
Anton Chekhov is supposed to have said something along the lines of “If you say in the first chapter that there is a rifle hanging on the wall, in the second or third chapter it absolutely must go off. If it’s not going to be fired, it shouldn’t be hanging there.”
It’s a sort of Occam’s Razor for writing: if it’s not necessary to the plot, it’s not needed in the book.
It’s a good rule and it turns out to apply in lots of different media. For instance, film:
“If Ripley’s going to use an awesome giant yellow robot exoskeleton to move crates around in the first act, she better be using it for a duel to death with an alien queen in the final act. If it’s not gonna be used to slap a screeching, drooling xenomorph in the face, it shouldn’t be there.”

There must be more of these (I guess James Bond is a prime example: Q always gives him a bunch of gadgets that turn out to be kinda handy later in the film. But the power loader one from Aliens is way more graceful, cos it’s just an incidental skill of Ripley’s).
Incidentally, wasn’t it kind of crass in Avatar when the climactic fight again turned out to be between a human in a robot suit and an alien, only this time it was the human that was the bad guy? And the suit was a fighting suit instead of just a warehouse tool? Pfff. Avatar.
Finally, TV Tropes has a good write-up of Chekhov’s gun, here. It’s actually an amazing site; I should post some more bits of it from time to time.