The northbridge takes care of front side buss communications between RAM, AGP and your processor. The southbridge takes care of onboard components such as network interface, sound, serial and parallel ports. They can become just about as hot as your CPU. The cooler we can keep these chips, the longer your motherboard is going to last. Also with better cooling you could overclock your front side bus at higher clock rates than normal.
*Fact: Microsoft Window's Blue Screen of Death vs Computerforums.org's White Screen of Death. Which is worse?