I have seen and understood the purpose of charge pumps. Used for pumping voltage levels in case of RS232 transceiver IC's. But most of the motor driver IC's also includes a internal charge pump circuitry. What is the exact purpose of this? As voltage required for the motor is already been given through external source.
You have to drive the gate of the N-channel Power FET in the driver chip high enough to make the FET switch fully on. Since the chip is using N-Channel MOS-FETs in the high side of the H-Bridge, you have to provide a voltage higher than the motor source to make the gate switch on. The chips include a charge pump to generate the needed voltage so that you don't have to implement it separately.
See also this question.
This isn't a very good answer, but should provide enough key words for you to find more information on your own - and maybe one of the resident experts will provide a more detailed answer later on.