Depends on the car.

Some cars do in fact have an amplifier. Other cars may use that wire to raise and lower the radio antenna. Others may use that wire to power an antenna amplifer. Or they might use a completely different pin for all of the above.

Part of your job as installer is to research and learn exactly what every wire does in your car's wiring harness. If you don't know, it's time to get a Crutchfield Master Sheet.