Look at the datasheet, section 7.2, the functional block diagram:
Note how the Feedback pin connects to a resistor voltage divider (R1, R2) which generates an error signal which goes into the Error Amplifier. This error signal is compared against a 1.23 V reference voltage.
If you add a resistor in series with pin 1 as shown in your schematic then this added resistor is simply in series with R1. That then changes the voltage division ratio of the voltage divider which now consists of (R66 + R2) and R1. That then means that the output voltage of the buck converter will become somewhat higher. So much higher that the + input of Error Amplifier again sees 1.23 V (same voltage as it sees on its - input).
It depends on which version of the LM2574 you have (3.3 V, 5 V, 12 V, 15 V or adjustable) what the actual output voltage will be. How to calculate the output voltage: Determine the values of all resistors in the voltage divider, take into account which version of the LM2574 you use, then calculate the division ratio Vout/Vref where Vref is the voltage at the input of the Error Amplifier. As Vref is always 1.23 V the output voltage will be Vref times that Vout/Vref ratio.
So: R66 in series with the feedback pin increases the output voltage of the buck converter.