Backup. Emergency spare. I always use my Arc4 when I need light for something. My Arc AAA sits on my keychain with a lithium AAA in case the Arc4 quits / breaks.

Besides, as Frenchy pointed out in another thread... what do you use to find your flashlight if you drop it in the dark? :-)

[Edit]
Don't underestimate the power of keychain sized flashlights. If you're dark adapted, the Arc AAA is more than enough to walk around a house or yard safely.