My fav job was an internship with a EW company that made these massive rackmount radar systems. My boss, the lead engineer, loved blue LED status lights. So the massive boards we made had hundreds on them. There were thousands total in the rack. GPIOs, busses, fpga signal outputs, all got an individual LED and mosfet.
It was crazy when it was running. You could visually debug a bus, if you wanted to.