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while the transistor is ON it would help as bottom part of the potentiometer , the upper one being the series resistor on base.
while in off condition , it would provide a termination for the base, as otherwise the base would be floating with a long wire and possible induction there of,
It shunts away any transistor base leakage current when the transistor is off.
When the transistor is on there will be a current through it of about 0.7V ÷ 4.7k = 149µA (for a silicon transistor). The rest of the current through the RB1 input resistor R will go into the transistor base.