Hi treez,

Thank you for using PI product on your design. The added resistor is to inject a minimum current on the EN/UV pin to achieve better stability. Since the series resistors R12 and R13 are connected to the rectified AC, there is a point (zero crossing) that no current will be injected to the EN/UV. Same also holds true, when no current is flowing at the forward LED of the optocoupler meaning the optocoupler transistor is open. Thank you.

Best Regards,
PI-SevenofNine