this is because rolling friction is lesser than sliding friction.this mechanism can be easily compared to rolling a ball and pushing a rectangular surface..imagine your car had no wheels and you had to push it..that would be so hard when compared to pushing a car with wheels...that's the main point that you have to notice in this mechanism.so the balls go over each other and thus the net friction on the whole mechanism is reduced which helps to perform your work faster...
They reduce friction because they roll rather than slide.
ball bearings have no edges they do not tip upon frictional surfaces instead, they roll and in effect, reduce friction
Think about a steel ball quickly rolling back forth in a metal track compared to a flat metal moving quickly in a metal track. 'Scraping' causes friction and becomes hot to the touch but the rolling will not get very warm. Therefore, items with ball bearings will last longer.
The reason that ball bearings are used is because it reduces the surface area of contact between two items. The ball also rolls easily as opposed to a flat needle-bearing. You will notice that a ball bearing generally utilizes either a grease or oil to reduce friction in cooperation with the naturally low resistance of the ball bearing, hence providing a overall low friction, low resistance way or turning around a shaft.
