Its a two pronged strategy. Firstly in advance of an event you reassure the public that you have a contingency plan in the event of things not being as they should but perhaps more importantly you are sending a message to the "silly brigade" i.e not the true fanatics but what I would call the lets have a shot at disruption idiots to say go away and leave us to deal with the real potential threat from well organised carefully planned fanatics. personally, I don't see anything disrupting the Olympics this time as I think in 1972 the Munich incident didnt do their cause much good.
The Olympics is seen as a peaceful event bringing Nations of the whole world together in sport. Disrupt that and consider will your cause or belief increase? Not sure. but better prepared always...