Human hands are soft, flexible, and can be cupped. Current robotic hands are hard and rigid. If a robotic hand with one thumb is used as a scoop to pick up small items (nuts and bolts or small Lego bricks) some of them spill off the side of the hand. (Though tilting the hand helps some.) The addition of a second thumb (see my blog of 18 August 2016) prevents this and makes for better use as a scoop.