LED-Indicators are not only useful for debugging, but also can display the current state of the circuit.

Following are two standard circuits for LED-indicators. On the right side you have your standard power-on LED. You can also drive it with microcontrollers or the like. On the left side is a driver circuit for 2pin duo-LEDs which can display two different states of your project.

If Sig_Input is low, T1 is off, letting current pass to the base of T2 (R2 and R4), turning T2 on. Current is flowing through R2 and D1 (red LED), passing T2 to GND.

If Sig_Input is high, T1 is on, pulling T2 off. Current is flowing through R3 and D1 (green LED) passing T1 to GND

With this you can for example make an indicator on power supplies for CV/CC. Or if you have something with PLCs you can display RUN/ERROR or something similar.