The circuit is designed to drive an external load. A fault condition in the external load circuit could feed excessive current or voltage back into the line drive circuit. If excessive voltage appears from the load, the two zener diodes will clamp that voltage to a safe level, which in this case is 10 V.The current in the zener diodes, op amp, and the remainder of the circuitry is limited to a safe level by resistors Rl, R2, and R3. D1 protects the op-amp output stage from 10 V appearing across the clamp diodes under a fault condition.
The advantage of this circuit is that, although it`s designed as unity gain buffer, the same techniques can be applied to inverting, noninverting, or differential gain stages.