Without power all those hospitals are nearly useless. Sure there are backup generators but they only run the bare minimum and only for so long.

Disable the power grid and the affects will be catastrophic on any developed nation. All the food will be spoiled within a few days to a few weeks. No business will be able to run including gas stations. Most communication will be down.

The whole area grinds to a halt untill power can be restored. Do enough damage to take out the power for a week to a large city and the damage will be incalculable. Not to mention the lives lost in that time.