Question

Ball bearings are used in bicycles, cars, etc., because?

Answer 1

Ball bearings are used to reduce friction between the surfaces.

Answer 2

ball bearing are used in bicycles,cars etc to reduce friction between the surfaces and also because the effective area of  contact between the wheel and axel is reduced

Answer 3

Ball bearings are spherical in shape. When these move they are not dragged, they roll in any direction. With a little effort (force and energy) these can be moved. The friction force that causes these to move is ROLLING friction and is minimum as compared to static friction and dynamic friction.

So ball bearings allow any direction of movement, and the friction associated is least and the effort required to move them is also very less.  The Loss of energy in moving the ball bearings is also the least.