Why were the arms of T.rex so short?

Recent discoveries of new skeletons of T.rex have revealed that the arm, although quite short, was powerful and had two large vicious claws on the end. It has been suggested that they were used as grapples so that T.rex could hold onto its prey very firmly while biting large chunks of flesh off. Most arms are used for reaching things with as well as from grasping, clearly in T.rex the arms had developed a very specific grasping function and had abandoned any reaching function at all.