An engine is essentially an air pump. More air entering the engine increases the efficiency of the combustion process. This results in more engine power. Maintaining optimal, unrestricted air flow does not happen since it must pass through a filtering medium. The amount of air resistance varies depending the attributes of the filter such as the size and surface area.
Air filters are essential for internal combustion engines. They are required to remove solid particulates such as dirt and any other debris in the air. The air filter should let in as much air as possible to give the engine maximum power while still filtering out any particles which may cause damage.
Source: wikipedia.org, knfilters.com
