Basically the round was created to solve a problem that never existed. It was developed to bridge a supposed "gap" between the 9mm and .45 Auto even though both rounds have substantial power as it is. The round appeals to a certain number of semi-auto enthusiasts who are dumb enough to believe that the 9mm is a pee-shooter, yet are too intimidated by the .45 Auto, which only has slightly more recoil. Still, it is a great round for self-defense.
Me: And can you give me an example of a .40 S&W getting the job done?
Know-it-all: Ummm...no. But 9mm is still wimpy because I said so.
Data compiled from police shooting incidents put the .40 S&W at a 96% chance of stopping an attacker with one shot, proving it's value as a formidable defense cartridge. Cheaper than .45 ACP rounds, yet just as effective in getting the job done, the .40 S&W has seen great commercial success.