Hi Minu,
In every STP version including RSTP, the first and foremost function of a root bridge is to provide a fixed reference point towards which every other switch computes exactly one shortest path, thereby creating a spanning tree. You certainly know that the first tiebreaker when comparing superior and inferior BPDUs is the Root Bridge ID, and the second tiebreaker is the root path cost. If the root bridge was not identified, it would be impossible to compute shortest paths to it, and both these tiebreakers would be essentially unusable.
In STP, the root bridge was given additional responsibility - to originate a BPDU every 2 seconds by default which was further relayed by other switches. In RSTP, each bridge sends its own BPDUs but many fields in them (Root BID, Hello Time, Forward Delay, Max Age, Message Age) are either directly retaken or derived from the BPDUs sent by the root bridge.
Best regards,
Peter
