It has been described in 36.331 TS 3GPP, that paging could be used to inform UE about changes in BCCH information (in idle and connected mode).
My question is why do we need to send that paging if UE will discover that some changes has been done in BCCH via TAG value and then read the BCCH ?
What is the added value of the paging in this scenario ?
Power saving is the main reason. UE need not detect the TAG period. just receive paging message OK.
UE came to know if any changes have done in SIB1 in 2 ways:-
1.through TAG Value as u said, but its helpful when UE changes tracking area or any cell.
2. paging msg is not only have sent for tracking area changes its also used to send regarding changes of Emergency cases like ETWS and CMAS notification and paging msg also send realted to incoming calls also,
actually UE will read this paging msg in a periodic time, that perodic tym set by operator, eg: if operator set 3hour so will read paging msg in every 3 hours, this is done to save UE battery Power.
i hope ur some doubts are clear.