John Anderton: Any contractions?
Casey: Only the ones you give me.
Agatha: Murder.
Wally the Caretaker: I like you chief, you've always been nice to me. I'll give you two minutes before I hit the alarm.
John Anderton: Why'd you catch that?
Danny Witwer: Because it was going to fall.
John Anderton: You're certain?
Danny Witwer: Yeah.
John Anderton: But it didn't fall. You caught it. The fact that you prevented it from happening doesn't change the fact that it was going to happen.
John Anderton: Why don't you cut the cute act, Danny boy, and tell me what it is you're looking for?
Danny Witwer: Flaws.
John Anderton: There hasn't been a murder in six years. There's nothing wrong with this system it is.
Danny Witwer: Perfect I agree, but there's a flaw. It's human.
Answer: Because subverting the expectations of the viewer makes it more interesting. The audience (and potentially John) are set-up to expect bad things, which don't happen. Once the "bad thing" happened, the suspense would be gone and everyone could relax. Expecting something bad but knowing when it might happen maintains the tension.