Air flow is very important when using a computer or any other device that dissipates a lot of heat. This is because heat build up over time can severely damage parts or components of the device if the dissipated heat is not drawn away from them. In a computer for example, the CPU and Power Supply give off a lot of heat. That is why the CPU is usually mounted on a heat sink with a fan on top of it so as to allow the heat to be properly removed away from the CPU. Also, the Power Supply has a fan mounted to the back which pulls the heat out of it and also assists in air flow through the computer.
Having secondary fans inside the chassis is very important as they help to keep the air flowing, bringing in cooler air while taking out the hotter air. Some people put two extra fans inside the chassis of the computer. One is mounted at the front of the chassis to suck in fresh air while the other is placed at the back to pull out the hot air. Both fans work in tandem – as one pulls air in the other draws air out. This continuous movement of air helps keep the components in the computer cool and thus prevents any damage that may occur from excessive heat build up.