Friction is unwanted in any situation in which free and continuous motion of mechanical parts is necessary. Some examples include the moving parts inside of an engine, door hinges and water slides.
Frictional forces oppose moving objects. When friction opposes the moving parts of an engine, the engine is forced to expend more energy to overcome the frictional force. Friction also transfers existing energy into heat. This heat can damage working machinery and cause damage. Many mechanical devices are used with lubrication or grease on the moving parts to reduce friction.
Friction can also be caused by air or water flowing past a moving object. This friction is reduced through the development of streamlined transportation devices.