Laminar VS. Turbulent
The purpose of it...
It creates the object to go faster. Such as boats or cars, the Batmobile was meant for speed since the smooth surface creates air to travel quick, the object moves quicker also.
It creates the object to go slower. Such as trucks, the air moves and hits the surface of the angular object causing eddies and making the whole object move slower.
The sink is an example of both laminar and turbulence being used in real life. The water rushing down is laminar, as soon as the water hits the sink it creates eddies turning the flow into turbulent.