If you have a network running some large bit of infrastructure, that is what you use it for. You'll have to explain why there's any necessity to connect it to the wider Internet. Answer: there isn't. If you don't so connect it, it can't be hacked.
Of course people might try to do things on the cheap and not have their own private network. But I'd call such networks mandatory for e.g the railways, power.
What *are* you talking about?