That circuit protects the overdriven signals going into an amplifier.
Instead of a zener diode we use a transistor.
That way we ensure that the above the ordinary input voltages ( 1 volt r.m.s.) can be acheaved without distortion.
Also the use of a transistor limits the number of components used in this schematic.
You don't need any potentiometers or voltage dividers.
The total distortion at the output until 700mV r.m.s. is 0%.
Above that until 1V r.m.s. the total harmonic distortion can be 0.02%.