[frezik]

Chelyabinsk-type cases are exactly why this is needed. Asteroids big enough to cause mass extinction events are rare, and hardly worth talking about except in the extreme long term.

However, much smaller asteroids that could potentially destroy a city are more common. If it struck in the right place during a contentious period of international politics (say, the Cuban Missile Crisis), it could easily be mistaken for a preemptive nuclear strike. Of course, the radiation pattern would quickly disprove this, but such evidence is not likely to defuse the situation if there's a lot of hot-headed people running around.

In such a case, just knowing ahead of time that it's coming would prevent a catastrophic loss of civilization. That's not even counting the humanitarian effects of evacuating the people in the predicted strike zone.