Probably just to make them look scarier and more dangerous. Real pachycephalosaurs probably didn't have these. Actually, no definitive fossils of pachycephalosaur hands have actually been discovered so far, but it is known that they had rather stubby arms, and most likely had fingers and claws to match (though their relatives, the heterodontosaurids, did have well developed arms and hands with prominent thumb claws).
Also,
as I discussed once before, it is possible (and possibly not even unrealistic) that the three
Pachycephalosaurus were after Cera for food. If this is what Don Bluth & co. had intended, then it seems reasonable that they would equip the pachys with predatory armaments such as sharp teeth and large claws.
