It makes the windmill rotate.
Air allows all the light that falls on it to pass through it.
It helps in the movement of aero-planes.
Air is transparent and hence it allows all the light that falls on it to pass through it. Moving air is called wind and since air has mass, moving air has energy. This energy is by virtue of its motion. This energy is used to rotate the windmills. The energy of the wind is transferred to the windmill. The windmill is used to draw water from tube wells and to run flour mills. Windmills are also used to generate electricity.
Aero-planes are able to fly because air moving under their wings is strong enough to hold them up. The engines (propellers) in an airplane are used to make the air under the wings move at very high speed and thus make them capable enough to hold the aero-planes up in the air.