DRV10983: Zener diode

Part Number:DRV10983

Hi team,

Our customers are testing DRV10983Z, and have below questions about the Zener diode. 

1. What's the function of the diode? 
2. Datasheet recommends 4V Zener Voltage (3.3V Vreg mode). Can they choose a higher Zener voltage diode?
3. If not using sleep mode, can they omit the diode from circuit?

Thanks,

Miranda