Why do computer components have fans inside them?

Computers can have more than one fan in the case. The purpose of the fan(s) are to circulate air, drawing air in from the outside and blowing it through the power supply, over the CPU and other heat generating components to remove the heat they generate and carry the heat outside the computer box or cabinet. Without this, the heat buildup in the computer can cause components to overheat and fail leaving you with a computer that does not work.