A little more arm mobility. Thought last time I tested the arm actuator had much more influence than the hand. It's the same for IS mech, but you can't remove or add it. (on tabletop it also allows you to grab a dead mech arm, or a tree, and use it as a giant club - which is pretty awesome, isn't it?)
I've used mech limbs as weapons before in table top. It is awesome. Not easy to do because of the dice rolls but still fun when you beat someone with their own arm.