High voltage is used in transmission lines to reduce energy loss which occurs in long-distance transmission, reduce power transmission cost and improve the efficiency of power transmission¹.
Power loss in transmission lines is proportional to the current flowing through it.
The resistance of transmission lines itself contributes the most to the power loss.
The power loss in the lines due to resistance is called copper loss or I2R loss.
Lesser the current lesser will be the power loss. Hence during power transmission, the voltage is stepped up to reduce current.
For transmitting the same amount of power to a distance, the power loss will be more while transmitted at a lower voltage.