People started putting dimples on golf balls over 100 years ago after noticing that the scratched ones tended to travel further. The dimples usually number between 330 and 500, but why do we need them?
To answer this, we need to consider the aerodynamics of the golf ball. The flow of air round an object can be described as either laminar or turbulent flow. Put simply: smooth or choppy air flow. Laminar flow past a spherical shape will result in the separation of the flow behind the ball - the flow of the air will stream outwards behind the object, like the way that ripples spread apart behind a duck or boat on the water. Flow of air around a rough shape, like a dimpled golf ball, will be turbulent (little air vortexes form in the dimples). Although this can generate more drag, the air flow sort of sticks to the surface and separates much less easily. This means that dimpled balls overall cause the least disruption of the air.
In the picture, the smooth ball generates a large wake as it moves through the air. The air pressure in the separated area behind the smooth ball is much lower - air molecules are further apart compared to the air around and in front of the ball. The air will try to equal itself out and air will rush into the area of low pressure, exerting what is known as pressure drag on the ball. You can imagine it like the ball is being sucked backwards into the area of low pressure, slowing it down. The dimpled ball separates the air less and has a smaller wake. The air behind the ball does not differ in air pressure as much as with the smooth ball so the dimpled ball has a smaller pressure drag and can travel further.
So why not make race cars and planes dimpled? Well, race cars are designed to allow the air to flow around them rather than sticking to the surface as drag would slow them down. With a smooth, aerodynamic car, the air flows around the shape and the wake behind is not so big any way. The difference is that a golf ball is not very aerodynamic to start with. Sticking dimples in a car would increase the drag without altering the degree of air separation by much, so it isn't used.
