Transistor functionality in a given schematic
It lights the LED to show that both outputs are on and above around 8V or so.

Note that the base is at the common terminal - I will call this zero for now.

When the positive output exceeds about 5.6V, ZD1 (a zener diode) breaks over and conducts. If the positive output were at 15V, then there will be about 5mA of current in the emitter. (15V - 5.6V - 0.6V) / 1.8k

The LED cannot light up unless there is a negative voltage output though as there is no bias for the LED.

Once the negative output exceeds 5.6V (for ZD2) plus the LED minimum forward voltage, there is a current path and the LED lights.