[michaelt]

> If someone intends to bomb a plane, they don't announce it

In the heyday of IRA terrorism, they would regularly phone in threats shortly before a bomb went off.

It's much easier to claim responsibility for a bombing if one does so before the bomb goes off. And when one's aim is political pressure, one might prefer to target military/ police/ infrastructure/ politicians/ whatever with fewer civilian casualties.

Of course there were also plenty of bombings without phone threats, so make of that what you will. And in one of the most deadly bombings - https://en.wikipedia.org/wiki/Omagh_bombing - the warning didn't specify the location of the bomb, and people 'evacuated' towards the bomb.