re: Why does T. Rex have vestigial forelegs?
One can do a lot with short limbs. My daughters know a girl their age who had meningitis when she was 2 or 3 and lost the distal half of all four limbs - knee and elbow down. She is very independent, dresses and feeds herself, does her hair and makeup, drives, has jobs, was on the local swim team (without prostheses) - she does have prostheses but does an amazing amount without them.
This is with something that occurred during her lifetime. I'm sure something that evolved over millennia would have significant functionality.