ETWS is a kind of public warning system (PWS) to notify all the UEs in a specific area of emergency situation like Earthquake or Tsunami or something similar. The concept is very similar to Cell Broadcasting in WCDMA and GSM network.
In WCDMA, we used a special channel called CTCH (Common Traffic Channel) for this purpose, but in LTE we use a couple of SIB messages to periodically broadcast the warning information to all the UEs in a certain area simultaneously. The SIBs that carry the information about ETWS is SIB10 and SIB11.
When an emergency situation happens, network broadcast the details of the emergency via SIB 10/SIB11 and inform UE to let decode SIB10/11 by sending special Paging message. When UE recieves this Paging message, UE has to decode SIB10/11 and display the warning information on the screen.
In LTE, there are three main components getting involved in sending ETWS as follows.
i) Paging : ETWS Notification.
ii) SIB 10 : Secondary Notification
iii) SIB 11 : Primary Notification
Now coming to the main question why secondary notification is divided to segments, the reason is each segment contains different Information which will enable the mobile terminal to discriminate between segments belonging to different warning messages.