Why A Zener diode is operated in reverse breakdown, where the junction conducts once the reverse voltage reaches the Zener voltage and then holds that voltage nearly fixed even as the current through it changes over a wide range. That flat voltage-versus-current behavior is exactly what a voltage reference or simple shunt regulator needs: a series resistor absorbs the change in supply voltage or load while the diode pins its terminal voltage. Rated Zener voltages run from a couple of volts to a hundred or more, with power dissipation setting the current limit.
Watch out The negative resistance region belongs to the tunnel diode, voltage-variable internal capacitance describes the varactor, and constant current under varying voltage describes a current-regulator diode, not a Zener.
Zener = steady Volts, Varactor = varying C, Tunnel = negative R. Match the diode to its trick.