Ball bearings reduce the effective area of contact between the wheel and axle, which minimizes friction. This is achieved by allowing the axle to rotate freely within a set of balls, thus converting sliding friction (which generates heat and wear) into rolling friction, which is significantly lower. This principle is crucial in mechanical efficiency, as it facilitates smoother motion, thereby enhancing the performance and longevity of vehicles like bicycles and cars, essential concepts often covered in engineering and physics curricula at the UPSC level.