>One possibility: it allows a big hand to be able to be tucked next to the body while not in use.<
Here's a WAG, which I haven't been able to find mentioned previously in the dinolist archives. >IF< elongated feathers first appeared on the distal half of maniraptoran arms for attitudinal control (a big if, I realize) it would have been advantageous for the creature to be able to control the length of the lever arm (and therefore the turning moment) by extending and retracting that aerodynamic surface. In that scenario, a wrist that could be adducted ("folded sideways") would be advantageous in that it would allow the creature to maintain the aerodynamic surface roughly horizontal to the airflow as the elbow was drawn in towards the body. Hmmmm....