What is the purpose of extra parallel resistor R2
I've done that when I'm feeding a signal to a microprocessor to sense when the switch is pressed but also want the LED to light directly.

Although the switch will pull the signal to the positive rail OK the LED alone will not pull the line low by itself when the switch is open - as the voltage gets down to ~1.5V the LED will stop conducting. Resistor R2 pulls the signal all the way down to ground so the microprocessor can correctly sense the switch being open.