It keeps all your moving internal engine components lubricated, which prevents wear/fatigue. Most manufactures recommend changing your oil and filter at 10,000km, unless running a synthetic oil. Over time high engine temperatures cause thermal breakdown of the oils viscosity. Viscosity is basically the rating of the thickness of the oil in your engine. Once oils loose the proper amount of viscosity they are no longer able to properly lubricate the moving parts that are so crucial to your engine running properly. Your oil filter removes any foreign contaminants in the engines oil. Both your engine oil, and filter should be changed at proper intervals to keep the engine properly lubricated.