Here, R1 and VR1 connects with the base. Due to base bias, it will conduct current from the emitter to the collector. Simply the bulb connects with the negative to positive side like a close switch. It will cause the light the bulb(B). If S1 does not connect with the base, it doesn’t conduct any current from the emitter to the collector. Thus, the bulb nor light, as the bulb is never connected to a negative line. It’s like an open switch.