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.
My dad programmed minicomputers, which normally had LEDs on the front panel that showed the binary contents of the program counter, and maybe the current instruction and some other status indicators. I remember him saying that he could tell if his program was running normally by the patterns in the lights.
When I was reverse engineering one device protocol, I could see the message patterns like addresses and flags in a screen full of raw hex numbers, essentially dehexing on the fly. Closest thing to matrix-style seeing of data. If you stare long enough at something, your brain will start to see patterns and automatically extract more information. It's a little crazy to experience for yourself how adaptable our brains are.
If the system was used to control something it was obvious.
There would be some kind of main / idle loop. When this was running, you could see the pattern in the data buss and in the address buss. The loop of code would not be many instructions, they would all be together, so the pattern on the address buss would be quickly repeating.
If you had a slow peripheral, you could see the ISR getting called. It would be at a different address than the main loop, and you could see it flash periodically, like every time a line went to the printer.
As an operator of the system, you had a unique perspective of watching the program run as the system operated; if it was data processing it would not mean much but in a control system it was uncanny.
There were some telephone engineers I used to know who worked in telephone exchanges in the electromechanical era, and they could tell when they walked in if things were not working correctly by the sound...
When I got my first Apple II+ (a Brazilian clone) I didn't have money for a floppy drive, so I used cassettes extensively (I learned to... erm... backup games to tape). By the sound of the tape you could identify code and bitmap segments, code being more random and data being more "rhythmic". Pattern recognition is a wonderful ability.
The Centre for Computing History, in Cambridge, UK, has a computer made from discrete components that occupies a wall. It can be downclocked to a point where you can see the code making its way through the machine, fetching data, pushing it through the ALU, and on to either the register file or back into the main memory.
There are a number of things we can do that expose system status via a LED panel. Network traffic, IP x IP, IP x incoming port, core status, L1, L2, L3 cache faults per PID or core, and so on.
It was crazy when it was running. You could visually debug a bus, if you wanted to.