For not bleeding out when multiple T-rexs were feeding on a creature. A larger appendage could be eaten by mistake and then they would bleed out as health care was still a future idea for a future species.
> "This kind of quirky idea was that their arms were small enough that they didn't get in the way of all these guys fighting over food with giant mouths, so they weren't basically biting their own arms," says Krumenacker.