Why carry a firearm? Because I can't find a holster big enough to fit a cop. Seriously, one never knows when a member of the criminal class will show up and express a desire to rob or harm me. In general, one is not in too much danger if one avoids bad neighborhoods and doesn't engage in gang activity or sell illegal drugs. But, there are still occasions when some riff-raff will make it into good neighborhoods and areas.
The second reason is that, when confronted by a criminal, the immediate tactical problem is how to survive alive and unharmed long enough to call 911 and get a police response on the scene. A gun gives one more option. Too many goblins (i.e., criminals) are on PCP, crack, or methamphetamine. In that case, giving up the wallet, cell phone, and watch may not save you from harm. Also, some of these thugs may just hurt you in order to prove how tough and macho they are. I'm 62, with diabetes, high blood pressure, and high cholesterol. I'm no match for a 19-year old with even a bat. The gun gives me one more option, depending on the tactical situation.
Finally, it is like an Oklahom Highway Patrol (state police) officer once observed, "Why shouldn't the good folks carry a gun, the bad guys already do?"