Lotta reasons:
- Sometimes the preferred J-frame model is built for .357 Magnum, even if the shooter intends only to use .38 Special. Case in point: 640 Pro. Best personal defense J-frame ever built, I say; why deny oneself one just because it can handle .357 but you prefer .38?
- Sending the fastest, heaviest lead down range isn't always the best way to protect yourself. .357 has big flash, big boom and big recoil. None of these things is conducive to fast, accurate follow-up shots (and I just laugh at folks who think they'll only need one shot) -- and in low light conditions put you at a disadvantage. Why subject oneself to that when there is excellent .38 defense ammo out there that can and will get the job done while mitigating .357's detracting points?
- Revolvers that can shoot two kinds of ammo are twice as advantageous when one has different intended uses and/or during times of shortage.
When I was younger, I often measured myself against how much of a given thing I could take -- concussive calibers, physical tasks, booze and all the other sorts of "man tests" that at the time mean so much but actually mean quite little. Come to find somewhere along the way the question stopped being "how much could I take" and became "how little did I need" to achieve the same effect.
