Explain why the wheel is such an important invention.

Wheel is the most efficient object that can be used to utilize the energy in its rotational motion. The wheel experiences relatively less friction compared to flat objects and also provide a means to travel larger distance with the same force.
Also the axis of rotation of the wheel coincides with its center of gravity; therefore the effect of gravity in producing any external torque is minimized. With the help of wheels, one can transfer heavy load over larger distances which was seemingly impossible to carry earlier.