On the origins of the .38 Super: First there was the .38 acp Model of 1900 which Browning designed for the military. You can see a pic of that here and read a little about it...
http://www.coltautos.com/
These guns in .38 acp were adopted by the Army and Navy but used only a bit. The round sent a 115 gr. .38 caliber bullet downrange at about 1150 fps from a 5" barrel and about 1050 fps with a 130 gr. pill which was about the same as the 9mm would do 2 years later (9mm was introduced in 1902 in the Luger).
But the military was leaning toward a .45 caliber gun which they finally convinced Colt and Browning to do for them but that's another story. Meanwhile, while they worked on a .45 the .38acp grew in popularity as a sporting cartridge. It's growth was slow but it was some the U.S., after all, was a nation of revolver shooters.
When the U.S. military got the 1911 sales of the .38 acp gun began slowly to fall off. By the 1920s Colt was looking for ways to boost sales of it's 1911s which lagged without orders from the military to fill. So they developed the .38 Super. The Super did well.
Colt developed the .38 Super to sell guns, same as every gun manufacturer does with any new round introduced. They also had two markets in mind, one was as a sporting round and they advertised it as a hunting round. The other was for law enforcement and the military.
With the rise of gangsters who used the new fangled automobile to get around the country quickly, law enforcement wanted a more powerful round from their handguns. S&W and Colt were in the hunt. Colt hit first with the .38 Super and S&W hit with the 38/44 hand ejector and a few years later the .357 (which they originally thought would only sell as a hunting round).
Now the .38 Super was the most powerful round that could be fired out of any commercially made handgun with commercially made ammo at the time it was introduced if we go by factory ballistics, that is velocity and energy. (see Barnes, "Cartridges of the World" 10th edition pg. 285) Till the .357 mag was introduced it held that title. It remained the most powerful round that was shot out of any U.S. made semi auto till the 10mm was introduced. It might be true that the old black powder Colt with a 7 1/2" barrel handloaded could get 1100 fps or so but in the 1920s no factory was making ammo like that for the Colt SAA or any other gun in .45 Colt or .44 Spl.
Now I said the Super did fairly well. It was chambered in the Thompson sub machine gun, it did well from the jump in overseas military sales, it did just ok in U.S. law enforcement sales (actually it did about as well as the 1911 in .45 acp did with law enforcement in the U.S. which is to say not all that well at all). It was used by some in the FBI and the Border Patrol and particularly in the South West of the U.S. where a flat shooting round that hit hard at long distances and penetrated barriers was appreciated. But mostly law enforcement liked revolvers.
A wider variety of ammo existed for revolvers than for semis.
When law enforcement switched to semi's in the 1980s they went with the 9mm and bypassed both the .38 Super and the .45 (except for a few).
The Super languished till the rise of competitive shooting in the 80 and 90s where it dominated for about 20 years and has given some way to the .40S&W and the .45 and rule changes that hacked at it's dominance. It is still very popular in some competitive circles.
Anyway enough of this long windedness.
tipoc