It doesn't have to be an advance warning to be useful, though of course that would be ideal.
The day the earthquake struck, people evacuated their buildings and then had no idea what to do. The government did in fact send out mass SMS messages explaining what had happened and telling people when it was safe to go back inside, but for most people those messages arrived so late as to be useless. (In my case, the SMS came eight hours late.)
If they can manage to get the word out to everyone quickly, that could be very helpful. I personally didn't receive anything from this supposedly successful test on either my DTAC or True number, though, so I'm not sure whether their bragging is justified or not.