Transfer cases exist in modern vehicles to transfer power from the transmission to all four wheels. Transfer cases are only used in four-wheel drives.
In vehicles that only offer a two-wheel drive, power is generated in the engine, travels through the transmission and is sent to the drive-axle differential and then to the drive wheels. In a four-wheel vehicle, the transfer case is connected to the transmission and divides the power to both the forward and rear axle differentials.

