That’s a good question! Carnotaurus and other abelisaurs have ridiculously tiny arms:

Those two bones that look like the wrist? That’s the entire forearm.

There’s some indication that they may have been especially flexible, which could point to them being used in some sort of display or other intraspecific behaviour. It’s also quite possible that they just…didn’t really serve any function. They could just be vestigial - that is, an evolutionary “leftover” that just hasn’t had the pressure to evolve away (like kiwi wings, or whale hips).