Yes, there is a purpose to that, and that is any time when there can be interaction between two objects that are rather far away from each other, because it shows a hard limit on what kind of interaction is possible.
A light-speed round-trip to the ISS is basically instant. That means, real-time communication with the ISS is no issue.
A light-speed round-trip to the moon is ~2.5 seconds. That means, you will notice the delay, but with a bit of patience, almost real-time communication is possible.
Mars has a light-speed round-trip duration of 6-44 minutes. That means, real-time communication is impossible, but you can communicate at "email-speed". You can easily communicate multiple times a day. You can reasonably manually control a robot/rover that way, even though it will be moving quite slowly.
A light-speed round-trip to Neptune is around 8 hours. That means, at max three messages a day. Manually controlling a robot/rover is pretty much out of the question. It will need to be either quite autonomous or incredibly slow.
A light-speed round-trip to Alpha Centauri is almost 9 years. That means, if we somehow manage to get people there, there won't be any possibility to direct or govern them from Earth. Whoever goes there might be able to send a message 10 times in their life time if they are lucky.
A light-speed round-trip to the Galactic Center would be around 50 000 years. That means, even if we get a probe to go there at light-speed, humanity will most probably be extinct before we get a response.
And that's pretty much the limit to our theoretical sphere of influence. If we could theoretically manage to fly something to the farthest objects we know of (e.g. the galaxy GN-z11, which is 13.39 billion light years away), they might possibly be gone by that time. Or actually, they might be gone already.