I, too, fail to see what possible use this could be. I've looked at the government website's FAQs on the topic and can only find this:
"Emergency alerts will be used to inform people about severe threats to life in particular areas, such as flooding or wildfires."
And this:
"However, these alerts are potentially life-saving so we recommend you keep them switched on"
I cannot think of any potential situation in this country where a text message from the government would keep me safe. Their blurb mentions "flooding and wildfires". In the event that either of those presents a threat, those affected would know about it long before somebody is dragged off their sofa whilst "working from home" to get an emergency text message out. Incredible as it may seem, people have learned to deal with emergencies and generally don't rely on the government to tell them one is about to occur.
But of course it make government ministers feel good because they've done something which will "potentially save lives." Something else they might consider (which would be certain to save a lot more lives) is to get the NHS abandoned in its current form and replaced with something more effective. But that is in the "too difficult" file.