Look at the DMZ map of Ukraine in your post. Look at the distances between Poland and the front lines. Ukraine's trains run mostly on electricity. By taking out power transmission infrastructure (note Russia is NOT bombing the 3 nuclear power plants held by Ukraine), Russia is making it VERY hard for Ukraine to move weapons, ammunition and troops.

I don't know if because civilians are affected if what Russia is doing is considered a war crime, but knocking out the power has a VERY definite military purpose.