Fans, or other air movers, are a critical part of any system level thermal management solution. There are cases where a fan is not preferred, such as in medical equipment (see our earlier post on that topic here), some military applications and in some consumer applications. And, fans do present the unwanted chance of mechanical failure being introduced into a system.
In a white paper written by ebmpapst, they note that:
Fans create a flow of air, which they can then force through equipment that needs cooling. The necessary output is determined from the flow rate and the required static pressure. If the airflow through the equipment is restricted to any great extent by built-in components or bends and junctions, then this will lead to a loss of pressure; the fan will try to compensate for this by an increase in the static pressure, though the fan will have to work harder than in free air operation. The characteristic curve of the fan gives precise information about the flow rate at specific static pressures.
Understanding these curves is critical and ebmpapst has a good, introductory white paper to help you understand them. You can get your copy of their quick and useful read by clicking: Fan Curves and Optimum Operating Points.