The speed of light is a physics topic, but I’d argue it’s relevant to most people working in IT: it’s the maximum speed information can travel at.
Anything involving networks is very much affected by it. The internet runs on fiber, where the speed of light is a fundamental constant! Even electricity in copper obeys those same limitations.
Similarly, anything involving staying alive is affected by the distance from earth to the sun. Doesn't mean that it's relevant for anyone interested in staying alive to know it.
However unless you work in high frequency trading or are trying to layout components for a GPU you can generally ignore speed of light delays below a couple hundred miles. You might develop an understanding for the rough travel time at light speed from New York to San Francisco, or Frankfurt to Sydney, but that doesn't translate to a good understanding how long light takes from this end of the room to the other end.
I think that’s mostly theoretical. There’s a lot of hardware on long connections that adds latency. The signal also may take fairly large detours (in some cases, it may even hop via a geosynchronous satellite. That adds about 70.000km to a trip, and another 70.000 for the return signal. For most users that is a thing of the past, though)
For example, when I ping microsoft.com, I get something around 100ms (Aside: what does that time mean? Round-trip or one way? I read a few man pages for ping, but couldn’t find out)
There also is the weirdness that, when pinging apple.com or google.com I get about 6 ms. They must be playing tricks with DNS.
It mainly acts as a lower bound. If your pings return 5ns, you know that answer cannot possible be correct. But I agree that's a rather constructed use case.
Anything involving networks is very much affected by it. The internet runs on fiber, where the speed of light is a fundamental constant! Even electricity in copper obeys those same limitations.