Transmission lines carry power long distances. High voltages (up to 765kV) are used because less power is lost as heat.
Steel towers can be spaced nearly a quarter of a mile apart. The cable is made with aluminum wire. Aluminum is a good conductor that is much lighter than copper. With lighter wire the towers can be spaced more widely so that fewer are needed.
The uninsulated cable can not be connected directly to the steel towers. Steel is also a good conductor and would give the electricity a path to the ground. The wire has to be hung from long stacks of glass insulators to keep the electricity separated.
At the top of the transmission towers, lightning arrester wires are connected directly to the steel tower so that lightning strikes will be grounded. Without the arresters, lightning could cause spikes, sudden sharp increases in the voltage that can damage transformers.
STEEL TRANSMISSION TOWER