If we use potentiometers as volume controls, don't they waste electric power?
If you do this, then it will waste a lot of power:



simulate this circuit &ndash; Schematic created using CircuitLab
You'll also need a very large and heavy potentiometer because it will have to handle all the power that the speaker can handle.
If you do this, then almost no power will be wasted:



simulate this circuit
The signal is very low power, and you only waste a tiny bit of that tiny bit by varying the volume.  The potentiometer can also be small because it only has to handle a tiny little bit of power.