bulb
The idea of the bulb in series is to limit the currentflow.
In case of a 100 watts bulb at 230 Volts you limit the current to about 450 mA.
for 110 Volts nearly 1 Ampère.
When you have the bulb in series with the variac it will glow because of the magnetising current of the variac.
When you test a power supply you don't need to use the variac but put the 100 Watt lamp in series.
Phase, 100 watt lamp, powersupply, Neutral.
Bear in mind when your powersupply draws more than 100 Watt the lamp will be at full brightness and you may have to put a 500 watt lamp or 1 kW heater in series to limit your current in the circuit under test.
Regards, Raymond