Setting the bow in the hand firmly can help reduce bow torque by providing better bow control. Bow torque is unplanned 'twisting' of the riser during the shot, before the arrow leaves the bow. Riser movement in the hand during the shot is bad for accuracy. Riser or limb movement after the arrow clears the bow is irrelevant.
When shooting at odd angles (up or down, bending, leaning out, etc.), a firm grip on the bow riser helps the shooter manage bow movement.