Replacing an engine air filter on a regular basis is an important and relatively easy step in maintaining a high-performing car over the long run and should not be overlooked. As its name implies, the engine air filter only allows clean air into the inner workings of the engine while keeping debris out. Although often microscopic, dust and pollen can build up over time and clog your engine if left unchanged.
The air filter itself actually functions much like parts of the human body, creating the highest surface area to volume ratio possible, simply by having a folded "accordion" structure. If you remember your high school biology, this is how the small intestine absorbs so many nutrients. However, unlike the small intestine, your air filter should be replaced every year. Dirty engine air filters can decrease engine performance and hurt fuel economy.
If you drive in congested or dusty areas, you may need to replace your air filter as often as every 5,000 miles, otherwise changing the engine air filter once a year should suffice. Replacing the air filter before a long road trip is never a bad idea either.