I was reading through a few movesets, particularly one for Greninja, and I repeatedly see this stated. Am I missing something?
This tactic can decrease an attack's damage on Greninja from 2x to 1/2x.
It may be a case of the opponent's Poke'mon being "heavy" most of the time...
Also with Protean it transforms into a Grass Type, getting STAB...
Hope I Helped!
Im pretty sure they just said it's good against Electric types in Greninja's case, because thanks to Protean, Greninja would be resitant to Electric moves after using Grass Knot.
Grass Knot is only good againt Electric types if they are heavy, and most of them arent, so this seems like the only explanation.