Honestly when I think about 357 Sig, I don't understand why it ever came into existence. After looking at some of my load manuals, I see given the right powder choice you can fire off 124 gr bullets at really high velocity in 38 Super. I saw one listed at 1500 fps. I didn't note the test barrel length, and I'm guessing it was long, but that velocity impressed me. I know 357 Sig is very capable, and can send a 9mm bullet zipping along pretty fast, but when you can get close with a 38 Super, I don't see the capacity trade off of 357 Sig making a lot of sense.
The sole reason is so that it can fit in a "short action" gun. 45acp, 10mm, and 38 super are all very similar in overall cartridge length. If a gun was designed around a 9mm or 40 S&W length round, often both the grip/magazine front-to-back length is too short and the length of stroke of the action is too shot to accommodate the longer rounds. And the converse is not completely true, but a little bit... it's usually a little more challenging getting a short round to run 100% reliably in a gun built/designed for a longer cartridge. It's doable, but it does take some engineering.
Sort of like "short action," "standard action," and "long/magnum action" bolt action rifles.