When powering up a circuit with capacitors on voltage supply rail inrush current may be high enough to deteriorate decoupling capacitors.
Inrush current is proportional to power supply slew rate :
I inrush = C × dV/dt
By limiting current (I inrush) we limit power supply slew rate.
Increase or decrease I inrush by changing the resistor value between emitter/base and GND.
