Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
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.