Final answer:
Extra fans in a computer's system unit enhance cooling via increased airflow and convection, preventing overheating and maintaining optimal system performance.
Step-by-step explanation:
Adding a fan to the front or side of the system unit in desktop computers aids in better air circulation and cooling. Although the power supply fan expels some heat, additional fans help with heat transfer by increasing airflow across the system's components. This improved convection cooling method pulls warm air out and brings cooler air in, much like a fan makes us feel cooler by enhancing the evaporation from our skin. Therefore, front or side fans help maintain an optimal operating temperature within the desktop, preventing overheating that can reduce performance or damage components.