What is the Most Common Application and Use of a Zener Diode?
The most common application and use of a zener diode is in voltage regulation.
Because a zener diode has such steady and constant voltage output across its terminals when its breakdown voltage is exceeded, it can act as a steady output voltage for a load connected in parallel with it.
For more information about a zener diode functioning as a voltage regulator, see
How to Connect a Zener Diode for Voltage Regulation.