Piltdown:
So what exactly was the job of the F15's? To shoot it down if she failed? Witness a human catastrophe? Put pressure on the crew? Answers on a postcard please.
Twofold:
1. To provide some measure of reassurance to the passengers that the diversion is being supported.
2. Once the captain and the F-15s are on the same page as to where the fight is diverting to, if the flight instead goes to some other plan, then the F-15 commander will be communicating as to "why are you doing that?" If there are no answers then the commander goes up the chain of command for instructions, possibly including shooting the airliner down.