It provides a path to ground for the base current. Otherwise, the transistor would be non-conducting; turned off.

 

use NPN BC 547 insted of BC 548 it will work

 

R2 turns on the PNP transistor. The 100k pot in series with the other 10k resistor do not do anything except reduce the base current slightly.

 

by increasing the pot to high resistance the voltage to the base is reduced.

 

this circuit looks familiar, is this an oscillating buzzer, it looks like a small amount of emitter signal/output is diverted back to base