The reason for non-smokers carrying lighters is easy to explain... lighting a woman's cigarette is the most sexual act between a man and a woman without doing anything, well, sexual.
But seriously, I carry a lighter for the same reason I carry a knife; you never know when you'll need it. Fire is so basic yet essential. Even though I have a small flashlight with me, sometimes i just use a zippo to light up a room. You can sterlize, or even just warm your hands on a cold day (for zippos, close the lid half-way while the flame is lit and warm the cover. Then close it and hold it between your hands). Butane lighters are also effective for these things.
Yeah, the reasoning is kinda simplistic, but it works for me.