They were issued to any soldier that had a job in which having the Garand would be a hastle, too big and heavy. For example, a platoon's radio operator had a Carbine. Engineers carried carbines. The mortar crew would have the Carbine. An NCO or an officer who had to carry maps and various organizational equiptment would likely have a Carbine.
It turned out to be used a lot more than it was intended for and the weapon it was designed to replace, the 1911-A1, well most people who carried a carbine also had one.
So in the end, it actually replaced nothing, just started a new breed of medium-powered, light-weight rifles. Helped out a lot of soldiers who already had enough equiptment to carry without the 10 pound M1 Garand on their shoulder.
Cheers,
Hans