Electrical power is carried by wires from where it is produced to where it is used. However, since all wires have some level of resistance, forcing a current through a wire takes some energy which is lost in the form of heat and not used by anyone. When less current is used, less energy is needed to force the current through the wire. For this reason, high voltages are desirable for "transporting" electrical power.

A wire wastes power because the resistance in the wire uses energy and the voltage must drop. The voltage drop is given by Ohm's Law which can be stated:
V = IR, or volts = amps * resistance

Since the equation for power is:

P = IV, and V = IR

so,

P=I (IR) = I²R.