With the voltage source connect as shown in the schematic, the zener diodes are reverse biased and will (try to) clamp the input voltage to about the sum of their individual zener voltage.
If for example D1 and D2 are a 4.7V zener and D3 a 6.2V zener, the clamp voltage will be about (4.7+4.7+6.2)V = 15.6V.
If the input voltage is standard 15V, the zeners won't be clamping.
If the input voltage would become 30V, the zeners enter the break down, clamping the the input voltage at 15.6V or bit higher, depending how much current is running through the zeners.
Depending on the output impedance of the voltage source, a very high current can run through the zeners. Therefore, a circuit breaker is added to prevent the zeners will burn due to a too high current. If the zener would get damaged, they likely fail open and the circuit will still see the 30V of my example above.
That is the reason that the circuit breaker is rated less than the (lowest) maximum rating of the zener diodes, so it trips before the zener diodes get damaged.
The zeners also serve a function as reverse polarity protection (as The Photon already pointed out). Then, the zener diodes clamp the (negative) voltage at about the sum of their individual forward voltage. Again, this could cause a very high current to run and the circuit breaker should act before the zener diodes get damaged.
Why a circuit breaker instead of a resistor?
A resistor could also be used instead of shown circuit breaker. When the input voltages becomes higher than the clamping voltage, the zener diodes will try to clamp again, causing a current. This current also flows through the resistor which causes a voltage drop, such that the input voltage minus this voltage drop becomes about the clamping voltage again (or a bit higher depending on the current).
A draw back of using a resistor is that it will always cause a voltage drop (depending on the load current), even when the input voltage is below the clamping voltage.
For that reason, a circuit breaker may preveal over a resistor.