Linear bearings are used primarily where something needs to be moved along a straight line with high accuracy. In other words, the object needs to only translate in one direction, and possibly move back to its starting position with high repeatability.
The implications for robotic pick-and-place are obvious, as are uses for optical test fixturing and calibration. The robotic uses of linear bearings have opened up a huge market for the devices in recent years. There is a plethora of manufacturers of the devices, so much so that standardization is sometimes a problem. The three primary types of linear bearings are ball, crossed-roller, and slide bushing.
Common Linear Bearings