We 'know' a roller coaster is safe, but if we completely believed that, if we were certain of it, half the thrill & fun would be gone as we careened around a banked corner at 60 mph feeling like the car was about to fly off the rails at any second.
We've watched the heroine get tied to the rails by the dastardly villain 100 times, and 100 times out of a 100 the hero has arrived just in the knick of time to rescue her as the freight train roared past -- and yet on the 101st time (if the film is well made) we're still glued to our seats with our hearts thumping and minds racing at the thought that, this time, the cackling villlain's many obstacles might prove too much.
How much less fun & exciting would 'Jaws' have been if the stoic & stalwart Captain hadn't wandered up wide-eyed and stunned and said "I think you're gonna need a bigger boat"?