function of zener between DS of MOSFET

Mosfets are notoriously vunerable to blowing up for no apparent reason...often voltage spikes. The zenner is probably there to try to prevent this.

I use a ludicously over-rated mosfet on all my stuff, yet on one product we still get the odd blow up....
The very high input impedance makes 'em particularly susceptible to EMI ESD type spikes on the gate or either power rail.
Hanging cloves of garlic around 'em doesn't appear to help.

Del

Is this an external diode? Or are you referring to the diode that's on the mosfet schematic symbol? The one on the symbol is inherent to the design of the mosfet - it's formed by the stackup of p and n type materials in the mosfet.

If you're talking about an zener added to the design, then that is, as del said, to protect the mosfet from switching transients. Is this mosfet driving a relay, motor, or other inductive load? That's the most typical time to use a protective diode.