Reverse engineering: Relay driver design consideration
In this configuration they are using the op-amp as a comparator. 

When it gets dark enough the LDR resistor gets higher which drops the voltage on the minus Pin of the OP_AMP. When it drops below the reference voltage from the other side of the bridge, the Op-Amps output goes to near the battery level, driving on the transistor on.

The RC circuit R9 and C6 form an integrator with a long time constant ~40seconds that stops small variations in light from causing the sensor to switch on and off around the ambient light transition time.

The op-amp is used this way to provide high impedance inputs that do not disturb that sensing bridge circuit.

However, the circuit could use some hysteresis. If it were me I would have added a positive feedback resistor in there from the output to the plus pin to shift the voltage on R7 up when the op-amp switches. Maybe 200K.

Why they used the peak holder in there instead of hysteresis is a good question... But then again, I have had some junior engineers that could never quite figure out how to do hysteresis... 

However, it may also be thy wanted the circuit to turn on and off close to the same light level, which they thought would not be the case with hysteresis.