You mean like the FitMario.pac or etc.? Those contain the attacks for your character, like one FitLucario.pac that makes his Aura Sphere black and super powerful, or one that gives Pikachu's taunt the ability to summon CDs.
Those are essential for new movesets, pretty much like the FitMarioMotionEtc.pac (and so on), which is in charge of animations and movements.
Unless you're talking about any of the other .pac files, like FitMario00.pac. Those are less essential than the FitMario00.pcs files. All you really need are those. .pac are needed for everything else.
Yeah, I mean the FitMario00's. I've noticed it's essential for transformers but some other characters have them anyways.