Purpose of Charge Pump in Motor Driver IC
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.