I think with the possibility of WW3 within the next 2-3 years, as well as more routine emergencies e.g. storms this will be set up to alert in the event of e.g. nuclear power stations being damaged or, god forbid, a nuclear attack.
People may have extremely limited time to seek shelter, hence the need for an immediate alert, rather than relying on people to check phones/internet, which won't be working anyway as so many people will be using their phones at once.