what is the purpose of these diodes?

Looking at an ST Micro part number 1NK60Z, I see that it has back to back zener diodes between the gate and source. What is the purpose of these?

They protect against excessive voltage on the gate input which could otherwise break down the gate insulator and cause a short. MOSFETS are very sensitive to overvoltage transients on the gate. Look at Table 8 at the bottom of page 4 in the ST data sheet.