>>16736283
I would bet (based on nothing but my own speculation) that they were probably proportionally larger in infant trexs and then just didn't grow along with the rest of the body. Maybe useful in self defense before size made claws irrelevant, or perhaps for newborn balance, or as a means of interacting with their environment.

Actually after checking the Trexs ancestor the Guanlong exhibited proportionally larger arms in juveniles.
Then again that's also true of most babies including humans, idk.

I don't think we actually have a (real) baby trex skeleton though.