What is the purpose of "MOSFET driver" IC's
A MOSFET driver IC (like the ICL7667 you mentioned) translates TTL Or CMOS logical signals, to a higher voltage and higher current, with the goal of rapidly and completely switching the gate of a MOSFET.  

An output pin of a microcontroller is usually adequate to drive a small-signal logic level MOSFET, like a 2N7000. However, two issues occur when driving larger MOSFETs: 


Higher gate capacitance - Digital
signals are meant to drive small
loads (on the order of 10-100pF). 
This is much less than the many
MOSFETs, which can be in the
thousands of pF.
Higher gate voltage - A 3.3V or 5V
signal is often not enough.  Usually 
8-12V is required to fully turn on 
the MOSFET.


Finally, many MOSFET drivers are designed explicitly for the purpose of controlling a motor with an H-bridge.