There really isn't a purpose for a tornado. They are the result of rotation in a supercell thunderstorm. They really do not clean the air as they destroy numerous things such as buildings and homes. The debris from these building and homes litter the air as the tornado goes through. They can also break or damage water and gas pipes. Thus leaking water or natural gas.
They are portals to the Outer Zone.
pressure equalizer., like a mini hurricane
It's God's way of getting rid of rednecks
It's God's way of keeping the redneck gene pool fresh by shuffling the trailer trash around.
It spreads different farm animals to different areas.
The purpose of any and all storms on earth is to equalize the earth's energy balance. Weather is driven by the unequal receipt of energy from the sun. Thunderstorms and tornadoes are efficient mechanisms to transfer excess heat from near the earths surface that is heated by the short wave energy from the sun to colder portions of the atmosphere. These currents are convective currents that are similar to those found in a pot of water heating on a stove. The stove heats the bottom of the pot and the convection currents take the warmer water and it rises to the top and mixes and warms the water through the pot.
In the atmosphere, the sun acts as the stove and the convective currents rise to dissipate the heat near the earth's surface and in doing so mixes the atmosphere.
The tornado is a special case of a convective current that has organized itself to be even more efficient in transporting the warmer air from the earths surface to the nearest colder air which is straight up. It takes a certain set of circumstances for a tornado to form and these parameters are continuously monitored.
All weather, wind, rain,sleet, snow are all caused by heat transfer functions using water and water vapor as a transport mechanism. And the purpose is to transport heat from areas that have too much (tropical regions) to colder areas (polar regions) in an attempt to make things equal. But, since the unequal heating will continue as long as the sun shines and the earth spins on it's tilted axis, it will never be achieved. So weather will continue.