These spark gaps are designed to protect metallic elements, like antennas, poles, pipes, roofing equipment... which are not connected to earth for operating reason, against the risk of flashover during a lightning strike on the installation.
When a large or exposed metallic element is not connected, for operating reasons, to the earthing system, it is better, in order to avoid destructive flashovers, to connect between the element and the grounded structure (or lightning conductor) a spark gap, which allows a punctual and a brief connection to earth during the lightning strike.
In stand-by situation, the spark gap insulates the element from the earth. During the lightning strike, the spark gap fires to create equipotentiality and to avoid destructive flashovers.