I agree with all the fire extinguisher, smoke detector, seat belt comments, etc. The handgun is the same type of protection -- ignored and invisible until you really, really need it RIGHT NOW.
The safe community argument also works.
I also believe that if I knew when and where danger was going to arise, I wouldn't go there, but since I can't, I take a precautions.
For me, though, the most basic answer is to protect my family. I have a 2 y.o. daughter, and we go for walks most every night when the weather is nice. I carry a .38 or .357 snub. We were walking on the sidewalk one evening when a large, menacing dog on the other side of the street came charging out of a darkened garage headed right toward us. He stopped when he got his paws a few feet onto the pavement -- just a street's width away. By then I had stepped in front of my daughter in her carriage, placed my hand on the butt of the revolver in my right front pocket, and begun to reach for my Benchmade folder with my other hand. Why should I carry a handgun? Maybe I could have fought the dog off with my bare hands -- I did it once before with a 110 lb. male and got a trip to the emergency room for my troubles. Maybe I could have done him in with just a knife if he continued charging. But what of he went past me for my daughter's throat or arm? And my punching or stabbing had no effect? Then what? Dial 911? That's what the gun is for -- to protect me and my family. A handgun would produce a loud noise and flash at the least, draw public attention, and at the most would kill the animal before it got within arms reach. At the very least, I would sacrifice a limb or some digits to occupy the dog while I shoved the barrel in its eye and emptied the cylinder.