We might easily be unprepared, but that the military tries things that might not work. Military attack is all about trying things that might cripple the enemy a

We might easily be unprepared, but that the military tries things that might not work. Military attack is all about trying things that might cripple the enemy and/or increase the cost of an effective defence. So an EMP isn't necessarily because it is expected to do horrific damage. It is just part of a thorough test of an adversaries preparations, making it harder to protect their infrastructure.