Answer:
Because we can't just let people be by theirselves. If we let them be, there would be much more chaos and destruction around us. And some people are also very timid when it comes to these kinds of alarms, people won't easily just listen to mere warnings no matter how dangerous it gets, so raising awareness of what's about to strike them will help them understand and/or be frightened to go to a safer area.
Hope this helps :)
