Bearing helps to avoid Moment at joints. If beam is rigidly connected to pier then Moment will occur, if moment occurs reinforcement and structural dimension will increase.
At the same time pedestal acts as a platform for Bearing. It also helps to provide vertical clearance between pier cap and bottom of beam. vertical clearance will be useful for maintenance work of Bearing. Incase Bearing to be replaced. Jacking to be done to rise beam. jacking will be provided between pier cap and beam bottom. without pedestal vertical clearance for jacking can't be achieved.