Originally Posted by cosmo kramer
So the only purpose of the PA is to avoid anyone panicking. The way to avoid that is to tell people what to expect.
Whilst I agree with
cosmo that a major reason for the PA is to inform the passengers, and hope that that information prevents panic, there is another reason!
The Captain is the only one permitted to give legally enforceable safety instructions, so "Be seated, and obey the instructions of the cabin crew" needs to be in there somewhere as a safety instruction from the Captain.
I usually add something along the lines of
"I know that this will disrupt your travel plans and apologise for that. Once we are on the ground I will be able to contact the company and give you some information about your onward travel arrangements."
This is on the theory that talking about such a mundane requirement AFTER the landing reassures the passengers that a safe landing is a given.