Every time a jet airplane flies through the sky, it creates two invisible tornados. They're not the kind of tornados that strike in severe weather. These tornados are called vortices and can cause problems - similar to the problems tornados on the land cause--for airplanes that may pass too close to the strong wind.
A vortex is formed by the difference between the pressure on the upper surface of the plane's wing and that on the lower surface. High pressure on the lower surface creates a natural airflow that makes its way to the wingtip and curls upward around it. When flow around the wingtips streams out behind the airplane, the vortices formed are strong enough to flip another airplane. Vortices also cause drag on the originating airplane, and that decreases performance and fuel mileage.
Standard procedure in air flight is to stagger the planes' flight patterns so that the vortices have dissipated by the time another jet passes through the area. That works, but since vortices can spread over miles, the gap between planes must be quite large. To create that large gap in air space, fewer flights are permitted to take off from airports. If there was a way to reduce or eliminate those vortices, more flights could be fit into the same amount of time.