Two use cases:
The driver is uneasy and focused on the road - the device is shaped to convey assurance
The driver is relaxed and not fully focused on the road - the device is shaped so that the hand can rest comfortably on top
The interface aims to help transition the user from scenario 1 to scenario 2 and eventually to a point where no such interface is longer necessary. In the end, the appeal of AVs for users is that they can focus on other activities while being safely transported from A to B.