One reason is the ability to carry spare ammo in spare magazines that fits both the sidearm and the long gun. (Like the old west. The Winchester lever action carbines were chambered in .44-40. Colt eventually offered the 1873 Single Action Army in .44-40 so cowpokes could shoot the same ammo in their six guns and saddle guns.
For me, another reason is that I shoot at an older indoor range. Rifle calibers aren't allowed, because they damage the backstop. Pistol caliber carbines are allowed.