We need bearings to support long shafts so that they do not buckle ( bend in between).
There are many types of bearings. Foot step bearings, simple bearings.
basically we use bearings to support the rotating shafts because if we directly support the shaft in a hole then due to the friction maximum power will be lost and handling of shaft will become difficult. So we need such an arrangement which can hold the shaft and also support the rotating motion without much loss in power transmission.