Transmission line is used to transmit electric energy from power plant to the users. To make it simple, a transmission line mainly composes electric wires or cables, electric towers, line fittings and insulators.

What is the use of the insulators?
Support: the insulators should support or hang the wires.
Insulation: the insulators connect the tower and the wires, to provide insulation to the wire.
 
In order to have these usages, what property the insulators should have?
Electrical performance: the insulators should have enough creepage distance to insulate, not flashover or puncture.
Mechanical performance: the insulators should have enough strength, tension and cantilever strength to let them support the electric wires.
 
Therefore, if the insulator is bad quality, then the whole transmission lines will cause problem, and loss will be much. The end user should choose the right insulators, and the manufacturer should control the quality.