The M1 Garand was the weapon of choice for infantry. The M1 Carbine, half the weight and with a less powerful cartridge, was the weapon of choice for support troops, and others not primarily involved in infantry combat. It was designed to meet combat needs less demanding than the M1 Rifle, but more than can be met by the M1911A1 pistol. It was more convenient to use than the M1, and less intrusive to their other duties, while still much more effective than hand guns.
The M1 Carbine: the War Baby. Baby because it was so much a 3/4 scale rifle; War Baby because it was the product of an urgently conceived requirement equally urgently satisfied by "Carbine" Williams and his colleagues at Winchester.
The Carbine was intended to "fill the gap" between the 9+ pound, full power M1 Rifle and the capable, but limited by its caliber, M1911A1 pistol. The intended user was the officer, the artilleryman, the signalman, the truck driver and the like, for whom the M1 Rifle was just too big and inconvenient to be practical, but who also needed a weapon with more useable reach than the pistol. At any but point-blank ranges the Carbine was easier to hit with than the pistol, too. More than 6 million Carbines were produced by a plethora of contractors, from hardware manufacturers to jukebox companies.
http://www.fulton-armory.com/M1_Carb.htm