The backup gun. Do I carry one? Not usually. Not when an extra magazine for my primary will give me an additional eighteen rounds of 9mm and take up less space than the six plus one of .380 I can get out of my Ruger LCP. What Mike in Avenger of Blood so lovingly calls an OSP—oh shit pistol—i​f you have to use it, you're in deep shit. If I'm in a situation where I've blown through the thirty-seven rounds in my primary and extra mag, those six rounds of .380 probably aren't going to do me much good.
So why do I have one? Because it's tiny. And I mean tiny. Which means it's concealable just about anywhere. In any outfit. Remember that scene in The Orphan where Anna entices a sex-maniac into a hotel room by way of her "sluttiest dress" with nothing but a three-inch Safe Maker strapped to her thigh? Well, she could have just as easily concealed a Ruger LCP under her dress and spared Spike the ensuing aneurysm.
But that wouldn't have been nearly as much fun.