Well, I guess it's useful if your attach it to your keyring. Then, if you drop it in the dark, in grass etc... you can find your keys more easily ...
Or if you want to be able to find someting when awaking in the dark of your room, in the middle of the night, without any light on.
i.e. you can attach it to your flashlight ...... <img src="images/graemlins/grin.gif" alt="" />

Alain