Hello Binoj,
the reason is to allow enough time to STP BDPU to propagate in all the network: STP BPDUs are sent every 2 seconds and 7 switch hops are supported by default in STP so 2 times 7 + 1 = 15 seconds.
STP target is to prevent bridging loops so before moving from one STP state to another one on a specific port this wait time is required in classic 802.1D STP.
Hope to help
Giuseppe