The two states of UE,RRC_IDLE and RRC_CONNECTED Used to monitor the system information.
if UE in RRC _IDLE it does
UE Control mobility
PLMN Selection,cell (re)selection.
By Using DRX Monitor the paging,sys info xchange
In RRC_Connected it does
network Control mobility
data transfer to/from UE
Acquire SYS info,
Perform measurements and give feedback to n/w
monitor paging,sys info chage,( ETWN / CMAS if UE Supports)
from this way UE always monitor the system information any take the resources if need