It allows a lightening strike to take an easy route to earth rather than causing damage or injury. It is made from a good conducting metal and can be found on tall buldings (quite often churches). The conductor would start high on the building and lead all the way down to the ground (earth).
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Little antenna like structures are on the roof (called air terminals) are connected with a heavy copper wire to a rod driven into the ground. The lightning strikes an air terminal and is conducted safely to ground by the copper wire rather than by conducting through the building and causing damage.
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It provides a low resistance path from air to ground.