The basic premise of a wheel and axle is to multiply the exertion placed on it. If power is applied to an axle, then the resulting power transferred to the wheel would multiply depending on the amount of power and the radius of the wheel. The result is movement of the wheel for travel.
This idea is applied to other items, such as a faucet or doorknob. Without the handles on either item, turning the internal rods to bring water through or to open a door would be extremely difficult. The handles act as wheels, whereas the internal rod mechanisms are the axles.
Wheels work well in physics, due to the fact that they can move objects without much force, since their lack of surface area friction is extremely limited. This gives the wheel an advantage when hauling heavy objects.