To add to other answers:

The enemy only has so many resources to target your resources. War has been, really for most of history, an economic struggle as much as a tactical one.

If they're planning an attack, they have to decide if the risk is worth the reward. They can't just magically hit all targets (or they can but that's because things go nuclear and hey, none of it really matters).

The more redundancies you have, the more likely you are to stop their attack, or to exhaust their ability to extend themselves. Given the absurd size of most road networks, that's a lot of potential targets to worry about, even if mostly undefended.