I know this technique has been discussed before, but peter_spy's recent comment on another thread about non-diegetic lights finally clicked with me. To verify my understanding, here's an example. I'm working on a mission that has a shop whose entrance is located at street level, under an extended second floor. The door is in the dark, located where the red arrow points.
There's a sign on the door that is completely invisible from this distance. Even when you get a lot closer to the door, you can't read the sign because it is in the dark - you have to deploy your lantern just to read it!
So I put a tiny light right in front of the sign, and now it is visible. You notice it, and you can actually read it from this location using your spyglass.
Depending on your screen brightness, it may not be very obvious from these screen grabs. But in game, it is very clear that there is something worth looking at in that dark corner. As peter_spy said, it draws your eye and focuses you in that direction. And even when you get close to the door, the 'lit' sign doesn't look fake, because it is barely illuminated.
Anyway, that's my understanding of non-diegetic lighting; hopefully I got it right.