The power grip of hand is due to the main function of

The power grip is used to do things like carry heavy bags or hold on to a handle. In the power grip, the object is held in the palm of the hand, and the long flexor tendons pull the fingers and the thumb so that they can tightly grasp the object. This grip is made possible by the four other fingers flexing (bending) and, most impoantly, the ability of the thumb to be positioned opposite the fingers. With the hand in this position, larger objects such as a stone or a heavy bottle can be held and moved in a controlled way. The heavier the weight and the smoother the surface is, the more strength is needed to hold and move the object. ref - pubmed.com