piper a lot of it depends what you are using it for.
A bolt and nut in its self is quite a complicated engineering system.
Then what you are actually using it for then comes into play.
There are huge books out there along with engineering design codes which will give you torque settings depending what you are doing with the bolt and nut. That's not to say that the torque is specified for no particular reason apart from that's what they use. They will give you a torque because there needs to be some conformity mainly to stop gorillas over torqueing and then making the bolt plastically deform or decrease its fatigue life. And also the threads need to be under a certain amount of load for the friction to work and the bolt not to undo itself.
In theory every bolt on a piece of machinery should have an associated torque specified for it. But in practise we don't for none load bearing items.