What does this motion of the air accomplish?

Answer 1

The sun is the driving force behind weather. As solar energy reaches the Earth, equatorial regions heat up more than the poles.

As land or ocean water warms, it heats the air next to it and this air begins rising. As the heated air rises, air from elsewhere flows in to replace it. If the Earth were not rotating on its axis, the air flowing in to replace the rising air would be coming from the polar regions. At the same time, the air that rises near the equator would be flowing high above the ground, to descend and replace the air that's flowing along the surface toward the tropics.

Earth's rotation causes the air to begin turning as it flows along the ground or high in the air. The result is a complicated picture with extratropical storms carrying air air toward the tropics and warm air toward the poles. Tropical cyclones such as hurricanes carry warm air toward the poles. Warm and cold ocean currents also help balance the heat budget.

source: http://www.usatoday.com/weather/tg/wglobale/wglobale.htm