I carry a Preon Revo SS on my keychain, (plus an Arc AAA UV whose battery I can cannibalize), I've got a IluminaTi with a lithium battery with my Leatherman in my backpack plus a spare battery, and if I'm riding my bike, I've got my 500 lumen bike light (Serfas) with spare battery (USB rechargeable). Plus my cell phone's camera flash LED.
When do I use them? All the time. I work nights except for one morning, so most of my waking hours are spent in the dark. My Preon gets constant use. A few years ago I used to carry a 1x123A light, but AAA lights these days are brighter than those were, and I find that it's enough for what I need. And if I need brighter, there's my bike light. The LED on my cell phone puts out of very useful, wide flood that isn't blinding, yet illuminates a large area well.
While useful for general use, flashlights and spare batteries are essential this time of year (well, in a few weeks anyway) in the desert, because during monsoon season, lightning causes power outages all the time.