Backwards compatibility is one of the reasons, meaning an humanoid form can interact with all the interfaces that humans already use in day to day basis, think

Backwards compatibility is one of the reasons, meaning an humanoid form can interact with all the interfaces that humans already use in day to day basis, think push/pull doors, grabbing two-handed objects or pushing a car; as well as the ability to be aid by other machines without wasting their own energy: think using electric stairs, elevators, boarding a car/bus/train, sitting down at a computer desk (so hands stay on while legs are off). Plus doing all that without the risks that spinning blades bring.