What is the purpose of the op-amp feedback circuit?
The OPAMP circuit does the current limiting. It is not a real current feedback, because the feedback path is alive only during over-current situation, and it can only reduce the output current.

The first stage is a difference amplifier - other call it as differential amplifier as well, but the latter is used more broadly - what converts the differential voltage across the 100Ohm series resistor into a single-ended one, then another OPAMP buffers it and feeds this error voltage back to the FB pin, if the diode opens. The diode rectifies this feedback, or with other words, it disconnects the OPAMP circuit from the FB pin under normal output current range. Not clear why is a zener diode drawn in the schematic. Any simple diode would do the job.

Why is it a current limit? The converter will push current into the output as long as the voltage at the output does not reach the desired value, which is set by the feedback resistors and a voltage reference. Note that they can not reduce the output voltage by sucking current back from the load. In the latter case, they would be called a generator or a driver. The output voltage is reduced by the load itself. Once the voltage on the FB pin is greater than the reference, the regulator thinks that the output is already too high, and it will not increase the output voltage, hence the output current decreases.