Even 9/11 was a regional disaster - in the sense that only a regional alert would have been needed.
9/11 was only regional in hindsight. At the time it was unfolding, we had no idea how many planes or targets were involved. In an unprecedented decision, all civilian aircraft in the entire country were grounded in a matter of hours. That certainly sounds like a situation where a national alert and broadcast could have been issued to warn the country what was happening or about to happen.
Because that large asteroid passed very, very close to the Earth yesterday, that certainly would be a reason for a national alert if it were actually on a collision course with the Earth.
You could have another multi-city terrorist attack, say, dirty bombs exploding sequentially in New York, Chicago, and San Francisco, and a threat from the terrorists that more are imminent.
You could be in the midst of a very bad, lethal pandemic or even a suspected bioterror attack in multiple sites across the country and the President orders all borders closed and all air travel to stop immediately in an effort to contain or slow the spread.
You could have another massive financial crisis which the government and the Fed aren't able to control and a real life "run" on the banks ensues eveywhere, so the President orders an immediate, nationwide "bank holiday".
Arguably, a "normal" nationwide broadcast could be used for any of these situations, but so could a nationwide EAS broadcast. Much of it may depend on the temperment of whoever is President at the time.