Regulators are good for several reasons. If the supply voltage to a stage sags in use then then the stage is drawing more current than the supply can continously supply, and this may be audible. A well designed regulator will also by definition keep noise from the power supply out of the amplifier circuits.
The regulator in effect acts like a giant capacitor to maintain the voltage and reduce noise.
The regulator in effect acts like a giant capacitor to maintain the voltage and reduce noise.