Scrappy is VERY useful for Normal types like Kangaskhan, because it allows them to hit Ghost types with their STAB normal type moves, which otherwise Ghost types are immune to. This allows the Pokemon, in this case Kangaskhan, to forgo dark and ghost coverage moves in favor of moves that allow it to hit rock and steel types supereffectively, as said types resist it's normal STAB. As all Pokemon with Scrappy are Normal types, they all enjoy this benefit. Scrappy is not always the best choice for ability, but it is a very useful ability nonetheless.
