There are more kinds of power than electricity generated by power plants, conveyed via powerlines.

Gasoline, diesel, natural gas, jet fuel… all have to get piped or trucked around, on regular schedules, otherwise a whole lotta stuff starts to break down rather quickly.

Whole lot of the infrastructure that makes that possible would or could also be seriously messed up by a full scale cyber attack.

… guess I could have said ‘fuel’, but fuel generally just is stored power in a condensed liquid or gaseous form, so, meh.