The air vents lead to your office where you basically don't want the animatronics to be, if they are in there they kill you and the game is over.
You actually explain this pretty well, the game does a really good job at keeping these secrets, like when the anaimatronics come to life at night.
It varies on which FNAF game I'm talking about, if it's 2 and 3, some of the cameras are legit inside the air vents.
You also make amazing points, the game's lore is pretty horrible. A man killed 5 kids and stuff them in the animatronics suits. Ever since this happened they are searching aimlessly for there killer, hence why they don't like adults.
I know this might be extremely confusing if don't know anything about these games...