Why does T. Rex have vestigial forelegs?
They can't reach it's mouth, so they are useless for feeding, and they are not truly vestigial, i.e., in the process of vanishing, because all bipedal dinosaur carnivores (like Allosaurus) related to T. Rex also have them, and they are similar in proportion to their bodies. There appears to be no progression of vanishing forelegs in the evolutionary sequence leading up to T. Rex (which I believe was the last of the line).
No, they must have some function common to the whole tribe. Perhaps they were used for gesture communication, or courtship signalling. or perhaps they were used to carry eggs or young to a new nest. Any ideas?