system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] What was Dumbledore using the Deluminator for before he bequeathed it to Ron?

The Deluminator does two things: captures and stores all the the sources of lights around it, and acts as a homing device allowing you to zero in on and listen to them even through protection spells.  

One of those things seems way way more useful to a man like Dumbledore. What'd he make it for that he needed that combination of magic abilities.
gpt
Nothing at all says that the Deluminator functioned as a... whatever it did for Ron all the time.

We only ever see Dumbledore use the Deluminator to turn off lights, so it's quite possible that he changed the Deluminator before he gave it on.

As for what he used it for... Turning out light when he didn't want to be seen.