In short, to maintain synchronization between UE and eNodeB .If any system configuration is modified within the system, UE should read and apply it. An UE may be in RRC-IDLE or RRC-CONNECTED state with respect to eNodeB.
As far as I know, UE re-acquires the system information in the following scenarios:
1. SystemInformation has been modified.
2. ETWS primary and secondary notification i.e. SIB10 and SIB11 respectively.
3. CMAS notification i.e. SIB12
In the first case, when any existing system information modification is get notified, UE waits till next system modification and then acquire the intended system information. But in case of point 2 and 3, UE doesn't wait till next system modification period, it re-acquires corresponding SIB(s) immediately.