That's not the point. People don't really seem to get that Russia is basically a terrorist state right now. The goal is not only to cut power but to terrorize - striking a nuclear plant and preventing firefighters from intervening is, as you currently see, scary as fuck. They're sending a message.

If you want an actual explanation for the extension of NATO into the former communist countries, look no further. East Europeans have a collective memory about how the Russian military operates.

If in future you want to force your new vassal state to buy your gas, you disable their nuclear plants.