Probably less than 3% of the population carries a firearm regularly (and that is a SWAG)
Given that, is there an advantage to looking like one of the 97% who are helpless, or is there an advantage to looking like you may be a problem?
It works the same as having visible bars on the windows, a "Guarded by XXX Company" sign in the yard, a dog food bowl with "Killer" left on the front porch and a host of other things. If you can make it look difficult, most robbers will pick an easier target - same reward, less work. That's just smart thug-thinking.
Why do officers wear easily identifiable uniforms? Drive distinctively marked patrol cars?
Wouldn't it be ~more effective~ to deterring crime if you couldn't tell if the guy next to you was a cop on patrol or just another guy walking on the sidewalk? You'd be worried that every casually dressed male that wasn't wearing a wife-beater was packing a badge.