The simple answer is, performance. .357 SIG in a 4" barrel has no trouble consistently exceeding 1400 fps with 125 gr bullets, with many (not all) brands of ammo. Velocity is more consistent, accuracy quite good. More velocity = better penetration, more reliable expansion, better long range performance. People switching from 40 S&W to 357 SIG often report better accuracy with 357 SIG.