Motor oil is the lifeblood of the engine, which needs transfusion on a regular basis.
As a car is driven, the levels of contaminants in the motor oil constantly increases. The motor oil should be changed before its contaminant level reaches the point where engine damage can result. Many OEMs recommend changing your oil every 3,000 miles, or three months, if you drive in severe conditions, which most people do. Check your owner's manual for specific recommendations on your auto. I have heard people recommend 7,500 miles between oil changes.