The relationship between power, voltage, and current is why high voltages are used to transmit power over long distances.
Anytime current (measured in amps) flows through a wire, part of the power is lost as heat. The greater the current, the greater the loss. That means that for high currents we need thicker, heavier wires to keep the losses down to a reasonable level.
By using higher voltages, we can send the same amount of power (watts) by using a smaller current, and thus don't need such large cables. To take a very basic example, assume you needed to transmit 96,000 watts of power from one point to another:
96,000 watts / 240 volts = 400 amps
To send that much current any appreciable distance without suffering huge losses would entail
gigantic wires. It's just not practical.
However:
96,000 watts / 2400 volts = 40 amps
That's much more manageable with reasonable size wires. Go up another magnitude of 10 and you get:
96,000 watts / 24,000 volts = 4 amps
That's why very high voltages are used to distribute power long distances around the country and around your town, and the power is only stepped down to 120/240V when it's a short distance from your home.
Just beware that the basic watts = volts x amps formula holds true for things like lamps and simple heating appliances, but things start to get much more complicated when it comes to motors and other devices (just ask your family engineers to explain power factor
).