Why are ball bearings used?
Ball bearings are mainly used to reduce friction associated with rotational motion. Ball bearings also support axial and radial loads and are typically used in applications where the load is small.
Ball bearings get their name from the use of balls to transfer the load. These balls are contained between two concentric wheels or races. One of these races is stationary, while the other is connected to rotating shaft. The rotating race causes the balls to rotate as well and due to the small contact area, this motion has a low coefficient of friction.
The ball bearings are used in a wide variety of applications and can be anywhere from as small as a rice grain to as large as one meter in diameter (for industrial applications). They are ubiquitous, used in everything from cars and fans to hard disks.
Hope this helps.