The primary purpose of a Zener diode is to:

A Zener diode is a special type of semiconductor diode designed to reliably allow current to flow backwards when a specific set reverse voltage, known as the Zener voltage, is reached. Because it maintains this specific voltage drop across its terminals over a wide range of reverse currents, its primary purpose is to regulate or maintain a constant voltage in power supply circuits. Distractors like boosting power supply voltage or providing phase shifts are incorrect because diodes do not amplify power or alter the phase of alternating signals in this manner. Standard diodes simply provide a one-way path for current, but the Zener's unique feature is its stable reverse breakdown behavior.