Protecting ADC Input inrush current
They don't show the I/V curve for the breakdown region in the datasheet, but for a diode in zener mode, it breaks down very fast as the voltage is increased (Vz in diagram below)

Source: https://www.electronics-tutorials.ws/diode/diode_7.html
The Iz point means that they set the current to 5mA and then measured the voltage Vz at that point. The intended application is more for setting a voltage so if you wanted to get a specific Vz (for a specific model) then set the Iz to 5mA, increase the current beyond 5mA and you get a cliff. So use a current limiting resistor before the zener to keep the current below the absolute maximum ratings of the diode.
You don't necessarily need a resistor, if the highest potential current that the zener would ever see was low enough to keep the part from over heating ( like from an opamp that could source up to 80mA on the output) then a 5V diode would dissipate P = 0.08*5V = 0.4W which is just under the max dissipation for the part which is 0.5W