Why A potentiometer is a variable resistor with a movable wiper that taps off a portion of the resistance, so turning the knob changes the voltage divided down to the next stage. That is exactly what a volume control does: it varies how much of the audio signal is passed along. The name comes from dividing a potential (voltage).
Watch out A fixed resistor has one unchangeable value, so it cannot adjust anything; a power resistor is just a fixed resistor built to dissipate heat, and a transformer couples AC between windings rather than providing a knob.
Pot = knob you turn. Potentiometer for volume, fixed resistors don't move.