In case of, say, fire at a chemical plant, it may be important that people downwind get notified right away, there might not be very much time to get inside, cl

In case of, say, fire at a chemical plant, it may be important that people downwind get notified right away, there might not be very much time to get inside, close windows etc. Same for collapsed dams, radiological accidents, that sort of thing. Sirens can do that very well. Other channels are much less reliable (people might not have a news website open on their computer, might not be looking at their phone to catch that notification, etc.) Radio is important because it's so widespread and low-tech. Pretty much every car has a receiver that can run for ages, there are inexpensive emergency ones that can be powered by crank and solar, so even in prolonged power outages, as long as the stations remain powered, and someone in your neighbourhood has a working receiver, it's still useful and pretty low-cost, since the infrastructure is already there.