The .40 was developed after the FBI sought a handgun and cartridge combination that could duplicate or exceed their +P .38 loads but in a semi-auto. They began to adopt the 10mm, but the reduced case volume and recoil of the .40 would allow it to work in 9mm-sized frames and make it more controllable.
I carried a GLOCK 22 with Winchester Black Talon cartridges for duty. It was amazingly effective, and I speak from firsthand knowledge. When subsequent tests determined (incorrectly) the differences in performance between the 9mm and .40 S&W were minor, I was one of the last holdouts, carrying a .40 until retirement.
The truth is, the .40 S&W is perfect. It can launch 155- to 200-grain bullets from the same gun. It has the widest use (defensive pistol, camping gun, concealed carry, etc.) of any other handgun cartridge besides the .357. Some shooters use the .40 to make major pistol. I’ve known some people to carry a .40 for dangerous game, but that goes far beyond its intended design.
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