Bearings can be divided into several types, rolling bearings, centripetal bearings, ball bearings, thrust bearings and so on.
It's supposed to be support, which literally means support for the axis, but that's just part of it, and support is essentially the ability to bear the radial load. It can also be understood that it is used to fix the axis. It is the fixed axis that allows it to only rotate, but controls its axial and radial movements.
The consequence of motor without bearing is that it can not work at all. Since the shaft may move in any direction, the shaft is required to rotate only when the motor works.
Theoretically impossible transmission effect, not only that, the bearing can also affect the transmission, in order to reduce the impact on the high-speed spindle bearings must implement good lubrication, some have lubrication bearing itself, called pre lubricated bearing, and most of the bearings must be lubricating oil, is responsible for running at high speed, not only because of friction increases energy consumption, more terrible is easy to damage the bearings.
The idea of turning sliding friction into rolling friction is one-sided, because there is something called sliding bearings.