Why is optimal air circulation so important to a computer system?

It is important because it helps to prevent overheating inside your case. A processor runs very hot, A video card runs hot. That's why there are usually some fans inside the case. Although they do help it's best to maximize the cooling inside your computer. The lower temperature it operates at the more you can push it. I run my case open aired, maybe that's a step too far but it does help. Airflow is VERY important though, don't unplug your fans unless you want your components to overheat! There are some extra cooling fans you can buy at fry's or other computer components stores, called PCI fans. They boost the cooling a a little, that should help.